An Update on House and Senate Polarization

Below are graphs of the House and Senate means from 1879 through 10 July 2015. We computed the means from our Weekly CS DW-NOMINATE Scores.

In a previous post we showed that polarization is asymmetric and due to the Republican Party moving sharply to the Right after 1976. Several people sent us e-mails asking if this was due to the realignment of the South (the 11 States of the Confederacy plus Kentucky and Oklahoma [the definition used by Congressional Quarterly]) into the Republican Party. The answer is No. In the House and Senate graphs below we show Northern and Southern Republicans along with Northern and Southern Democrats. In the House the Northern and Southern Republicans moved in tandem during the entire post WWII period:

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In the Senate we only show Southern Republicans after 1961 because there were so few Republicans from the South before then. The Southern Republican Senators are more Conservative than the Northerners but beginning in the mid to late 1990s the two wings begin to converge. In contrast, the remaining Southern Democratic Senators are a bit more Conservative than their Northern counterparts.

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In terms of Party Polarization it is at a Post-Reconstruction high in both Chambers. The House Republicans are slightly less Conservative in the 115th but the House Democrats are slightly more Liberal. Hence, this produced a small uptick in polarization (so far) in the 115th House. In the Senate the increase in polarization is due to both Parties moving away from the Center. However, it bears repeating, overall, the increase in polarization in both chambers is primarily due to the Republican Party moving to the Right.

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More on Asymmetric Polarization: Yes, the Republicans did it!

In our last post we showed the Party Means for the House and Senate since 1879. We used our Weekly Constant-Space DW-NOMINATE Scores to do those graphs. Below is the graph for the House which shows the Republican Party moving rapidly to the Right after the late 1960s. In contrast, the movement of the Democratic Party to the Left is entirely due to the Southern Democrats becoming indistinguishable from the Northern Democrats. Indeed, the Northern Democrats have not moved since the 1960s.

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Not everyone believes the above graph. For example, Peter Wehner argued in a NYT op-ed that the “Democratic Party … has moved steadily to the left since the Clinton presidency.”

The purpose of this Blog is not to engage in debate with pundits. We use it to show the results of our 33 year research project on the history of Congressional Voting (note the discussion on page 14 of this 1983 Working Paper). Consequently, the purpose of this post is to show that Asymmetric Polarization is real and is not some methodological artifact of our DW-NOMINATE method.

Our first experiment is to run DW-NOMINATE just on the House for Congresses 1-113 using random starting coordinates to see if the asymmetric polarization observed in the modern era holds up. In particular, if a member’s coordinate on either dimension was negative we drew a uniform random number between -1 and 0 and inserted into the starting coordinates. Similarly, if the member’s coordinate on either dimension was positive we drew a uniform random number between 0 and +1. We then estimated the cutting lines for the roll calls using the Cutting Line Procedure in Optimal Classification. We used the Poole and Rosenthal Constant Model where each member’s ideal point is the same throughout his/her career in Congress. For the first five iterations we fixed the second dimension weight to 0.4 and Beta to 7.5. Iterations six through fifteen reverted to the normal DW-NOMINATE algorithm. Below is graph of the Party means of Houses 46 to 113 (1879 – 2014). Note that it shows almost exactly the same pattern as the figure above.

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Our second experiment is to run Optimal Classification on the House and Senate for Congresses 1 – 113 simultaneously using the 650 members who served in both the House and Senate as “glue” (bridge observations). Below is a graph of the Party means of Houses 46 to 113 (1879 – 2014). It is virtually identical to the CS DW-NOMINATE House graph shown above.

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Our final experiment is to run the Basic Space procedure (Poole, 1998) on W-NOMINATE scores for every House and Senate for the 75th to the 113th Congresses (1937 – 2014). This application is the same as that shown in Poole (1998, p.982-989). The graph of the Party Means of Houses 75 to 113 is shown below.

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The same pattern occurs in the Basic Space application to the W-NOMINATE scores but it is obviously not as “clean” as the other graphs. Nevertheless, the pattern is the same. The Republicans move to the Right while the two wings of the Democratic Party converge. The Basic Space result is very strong evidence because each W-NOMINATE configuration is scaled separately and the first dimension is constrained to range from -1.0 to +1.0. Hence, the only way that the Republican mean could be moving to the Right is through replacement of moderates by conservatives.

Below we reproduce a post we did on our old Blog in 2012. was hacked by Chinese hackers and completely destroyed in early March. All the old Blog posts were lost and we moved the Blog to wordpress for security reasons.

Polarization is Real (and Asymmetric)
by Nolan McCarty on May 15, 2012 · 14 comments
in Legislative Politics

This post is co-authored with Keith Poole, Howard Rosenthal, and Chris Hare. It is cross-posted at voteview blog.

The recent outburst of scholarly and popular interest in political polarization has attracted attention to the methods we use to measure this phenomenon. One frequently voiced concern (see a recent column by Sean Trende) is that Congress may not have polarized as we have claimed in publications and blogs stretching as far back as 1984. The concern is that the meaning of ideological (NOMINATE) scores are tied to the legislative and historical context of the roll call votes that are used to estimate them. For example, the content of roll calls votes cast by members of 90th Senate that dealt with the Vietnam War, civil rights, and funding for LBJ’s “Great Society” programs are quite different than those votes cast in the current Senate. Thus, being the most conservative Senator (with a score of 1.0) in 1968 would mean something different than having an identical 1.0 score in 2012.

Indeed, temporal comparisons should not be made for ideal points generated from static scaling methods. Static methods (like W-NOMINATE) treat each legislative session separately and there is no valid way to compare the scores of legislators from different years. However, we developed a dynamic methodology, DW-NOMINATE (McCarty, Poole and Rosenthal 1997), to allow for over-time comparisons of legislator ideological positions. The key innovation is the use of “bridge” legislators—members of Congress (MCs) who have served in multiple sessions—to compare the positions of legislators who have never served together.

A sports analogy to the overlapping cohorts method is the “common opponents” statistic. If we want to compare two teams who have not played each other, we compare their performances against a common opponent(s). Likewise, MCs who have not served together can be compared with the use of a “bridge” legislator who has served with both. For example, if we know that Sen. George McGovern (D-SD) is more liberal than Sen. Patrick Leahy (D-VT), and that Sen. Leahy is more liberal than Sen. Max Baucus (D-MT), then we can say that Sen. McGovern is more liberal than Sen. Baucus. Though intransitivities may arise cases involving 3 or more sports teams, Poole shows in his 2007 Public Choice article “Changing Minds? Not in Congress!” that MCs remain remarkably static in their ideological positions over the course of their careers. Thus, we are on much firmer ground in making over-time comparisons between MCs with the caveat that we cannot compare members outside of one of the stable, two-party periods of American history. For that reason, when we discuss current polarization we focus on the period from the end of Reconstruction in 1877 to the current period.

With the use of overlapping cohorts, we can make the over-time comparisons needed to analyze polarization. A good example is Senator Richard Lugar (R-IN), who, after his primary defeat last week, will have served in the Senate between 1977 and 2013. As David Karol points out, Lugar himself did not change very much over time: he was a reliable conservative who moved only somewhat towards the center during a 30-plus year career (from a DW-NOMINATE first dimension score of 0.348 to 0.241). DW-NOMINATE scores range (with slight simplification) from minus 1 to 1 or a band of two units. So in 30 years, Senator Lugar moved just five percent on the liberal-conservative dimension.[1]

For Lugar, what is more dramatic is the change in his ideological position relative to the Senate Republican Caucus. In his first term in Congress, Senator Lugar was the 23rd most moderate Republican in the Senate; in the most recent term (through 2011), he was the fifth most moderate. Even if he had maintained his freshman score of 0.341, he would still have been the 12th most moderate Republican in the 112th Congress. This repositioning occurred because almost every new cohort of Republican Senators has been more conservative than Senator Lugar. That fact is the basis for our claim that the Republican Party has moved to the right.

To be sure, political polarization is not entirely asymmetric. Congressional Democrats have moved slightly to the left during this period, but most of this is a product of the disappearance of conservative Southern “Blue Dog” Democrats. But the northern Democrats of the 1970s are ideologically indistinguishable from their present-day counterparts, with average scores around -0.4.

Though Democrats have not moved nearly as much to the left as the Republicans have to the right, they have also contributed to polarization, in our opinion, by embracing identity politics as a strategic tool. In Roosevelt’s New Deal, the Democrats advocated redistribution and regulation of business. These issues remain active to some extent, but with time emphasis has shifted to issues centered on race, gender, ethnicity, or sexual preference (Gerring 1998). This distinction, however, is not necessarily picked up in roll call voting. But it does represent an important rhetorical shift from the Roosevelt and Truman-era Democratic Party that likely worsens political and social divides.

Nonetheless, we should be careful not to equate the two parties’ roles in contemporary political polarization: the data are clear that this is a Republican-led phenomenon where very conservative Republicans have replaced moderate Republicans and Southern Democrats. Thomas Mann and Norm Ornstein do an excellent job of navigating these trends in their new book: It’s Even Worse Than It Looks: How the American Constitutional System Collided With the New Politics of Extremism.

Moreover, the rise of the “Tea Party” will likely only move Congressional Republicans further away from the political center. For example, the five Tea Party-backed Senators elected in 2010 (Senators Rubio, Paul, Toomey, Lee, and Johnson) have an average first dimension DW-NOMINATE score of 0.795. Moderate MCs (especially Republicans) are increasingly likely to be “primaried” out (e.g., Sens. Bob Bennett (R-UT), Joe Lieberman (D-CT), and Lisa Murkowski (R-AK), as detailed in a recent post on [The post was lost in the break-in and destruction of]).

The public policy consequences of polarization are immense. Bipartisan agreements to address looming issues like the budget deficits, spending on entitlement programs, and immigration are now almost impossible to reach. In contrast, during Ronald Reagan’s administration, about 40% of the members of Congress could be described as moderates. Reagan was thus able to forge major bipartisan agreements to cut taxes in 1981, raise taxes in 1982, fix Social Security (the Greenspan Commission) in 1983, and pass immigration reform (which included amnesty) and major tax simplification in 1986.

As shown in the second pair of figures below [see Figures above], only about 6% of Representatives and 13% of Senators in the 112th Congress can be described as moderates (defined as having a first dimension DW-NOMINATE score between minus 0.25 and plus 0.25). This absence forces major legislation, such as President Obama’s health care package, to be passed by one party. But unlike major bipartisan efforts (e.g., the Social Security Act of 1935, the Civil Rights Act of 1964, or welfare reform in 1995), legislation passed by one party is less likely to earn popular acceptance (as evidenced by the partisan breakdown in opinion on “Obamacare”).

Polarization is real. Arlen Specter was reelected to the Senate as a moderate Republican in 2004. In the 2010 election, he was replaced by Pat Toomey. Do academics and pundits really want to argue that Republicans have not moved to the right and that Pat Toomey might be more moderate than Arlen Specter because the congressional agenda has changed? Let’s not get picky about polarization. It’s for real, and it is making the United States dysfunctional.

1 Legislators’ DW-NOMINATE scores are allowed to move as a linear function of time, while a single coordinate is estimated for each legislator with the Common Space procedure; methodological issues aside, the linear and constant methods produce yield the same pattern of contemporary political polarization.

Gerring, John. 1998. Party Ideologies in America, 1828-1996. Cambridge: Cambridge University Press.

McCarty, Nolan M., Keith T. Poole, and Howard Rosenthal. 1997. Income Redistribution and the Realignment of American Politics. AEI Studies on Understanding Economic Inequality. Washington, DC: AEI Press.

House and Senate Party Means 1879 – 2015

Below we use Weekly Common Space DW-NOMINATE scores to plot the means of the House and Senate Democrats and Republicans 1879 – 2015 (May 31).  The Democrats in the House for the first time since 1921 are homogeneous across the North and South (the 11 States of the Confederacy plus Kentucky and Oklahoma).  The Republicans in the 114th House have almost exactly the same mean as they had in the 113th House so we may, at last, see a leveling off of polarization.  Indeed, the slight increase in polarization in the House from the 113th to the 114th (see below) is due to a slight shift to the left of the Democratic Party mean due to the Southern Democrats shifting left.
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In the Senate the Southern Democrats are more moderate than the Northern Democrats but both groups have shifted to the left.  The Republicans have shifted to the right.  This is why the uptick in polarization in the Senate is larger than the House (see below).

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The results indicate that polarization (i.e., the distance between the two parties on the first dimension [Liberal vs. Conservative]) has increased slightly in both the House and Senate.  In an earlier post we thought that the polarization in the Senate had dropped a little.  That was an error.  We used the wrong dataset.  The figure below is correct and uses all the roll calls through 31 May 2015.

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I bears repeating that the trend to polarization has been largely due to the Republican Party moving to the right since the 1976 elections. In contrast, the Northern Democrats in both Chambers have barely budged in the past 40 years. What has happened is the the Democratic party has become much more homogeneous.

Ideological Dispersion of the Parties in Congress

A recent New York Times Magazine article examined ideological differences between the progressive and centrist wings of the Democratic Party, using the potential Democratic Senatorial primary race in Maryland between Reps. Donna Edwards and Chris Van Hollen as an example of “the great Democratic crack-up.”

Below we use DW-NOMINATE scores to illustrate the ideological positions of Democrats and Republicans in the current House and Senate (Note: these scores are from a new stand-alone Common Space DW-NOMINATE program that can be run weekly as new roll calls are cast; data available here.)

In the present (114th) Congress, it is clear that Republicans occupy a wider swath of ideological territory than Democrats. It is easy to identify distinct ideological clusters of Republicans, but not for Democrats (a point we make here). The distance between, for instance, Republicans Senators Lisa Murkowski and Ted Cruz dwarfs that between Democratic Senator Elizabeth Warren and former Senator Hillary Clinton. In fact, the distance between Democratic Representatives Donna Edwards and Chris Van Hollen is more comparable with the distances between Senators Lisa Murkowski and John McCain or between Senator McCain and Representative Paul Ryan.

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The images below show trends in the ideological dispersion of the parties in Congress since 1879 by plotting the variance in DW-NOMINATE scores among House and Senate Democrats and Republicans in each Congress. Higher values indicate greater ideological dispersion within the parties in both chambers.

From a peak in the mid-twentieth century (when the Democratic Party was split between its Southern and non-Southern wings), the ideological variance among House and Senate Democrats has steadily declined. Congressional Democrats are now more ideologically unified than at any point since the early 1900s. The ideological variance of Congressional Republicans has been flatter over the last century, but there is some suggestion of a jump in dispersion among Senate Republicans in the 112th Congress (following the 2010 midterm elections).

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New Estimates of Polarization in the 114th Congress

Below we use Common Space DW-NOMINATE scores to compare polarization in the current (114th) and previous Congresses. Common Space scores permit comparability across time and between the House and Senate.

The distribution of the ideological (first dimension) scores of the House and Senate Democrats and Republicans in both Congresses are shown below, with the mean scores of members in both parties and chambers marked in the plot.

The results indicate that polarization (i.e., the ideological distance between the two parties) has increased slightly in the House, but has decreased slightly in the Senate. The entry of freshman Republicans such as Senators Mike Rounds (R-SD) [with a score of 0.189], Ben Sasse (R-NE) [0.277], and Dan Sullivan (R-AK) [0.285] has moved the Republican Senate mean slightly to the left in the 114th Congress. In the House, both the Democratic mean and the Republican mean have moved away from the center, but only slightly. The long-term polarization trends in both chambers are shown in the second plot.

These images are from a new (nearly) stand-alone Common Space DW-NOMINATE program that can be run weekly as new roll calls are cast.

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Congressional Policy Shifts, 1879-2014

Below we use first dimension DW-NOMINATE scores, which represent legislators’ positions along the familiar ideological (liberal-conservative) spectrum, with lower (negative) scores indicating greater liberalism, and higher (positive) scores denoting conservatism, to plot the chamber means for legislators and winning outcomes on roll calls for Congresses 46 to 113 (2014). These are updates to Figure 4.1 (page 60) in Poole and Rosenthal’s Congress: A Political-Economic History of Roll Call Voting and Figure 4.1 (page 80) of Ideology and Congress.

We see that the overall chamber means (plotted with red squares) remain mostly stable over time, a reflection of a competitive two-party system. Of the two chambers, the House mean has shifted more to the right in the period following the 104th Congress (the 1994 “Republican Revolution”). However, the position of the mean winning coordinate in each chamber has proved much more volatile, particularly in recent Congresses. This reflects the frequency of party-line votes between rival partisan coalitions that have moved steadily apart in recent decades.

Consequently, the mean winning coordinate–-which is an approximation of the ideological location of policies enacted in the chamber–-has diverged from the overall mean chamber score in both chambers: to the left under Democratic control, to the right under Republican control. Indeed, the mean winning coordinate in the 111th Senate (a session in which the number of Senate Democrats fluctuated between 57 and a supermajority of 60) was the furthest to the left since the 75th Senate during the Great Depression and the New Deal. Further, the mean winning coordinate in the 113th House was the most conservative since the late nineteenth and early twentieth centuries, following the realigning election of 1896, after which Republicans controlled the House for 32 of the next 38 years. Conversely, the mean winning coordinate in the 113th Senate was the most liberal since the late nineteenth century.

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An Early Look at Polarization in the 114th Congress

Now that the 114th House has conducted 66 roll call votes and the 114th Senate has voted 53 times, we now have enough data to take a very preliminary look at ideological polarization in the new Congress. Following standard practice, we use first dimension DW-NOMINATE scores as measures of legislators’ liberal-conservative positions.

The first two plots below show the mean score of Democrats (Northern and Southern) and Republicans in both chambers over time. Though we are hesitant to put too much stock in these results at this early date, it looks like polarization (the ideological distance between Democrats and Republicans) is on pace to take a slight uptick in the 114th House but may level off in the 114th Senate. In the Senate, this is due to a slight shift in the Republican mean back to the center. Some of this may be due to the exit of Senator Tom Coburn (R-OK) [whose score is 0.792] and the entry of Senators Cory Gardner (R-CO) [0.216] and Mike Rounds (R-SD) [0.398]. There may also be a procedural aspect as Majority Leader Mitch McConnell (R-KY) has allowed several votes on amendments.

Interestingly, the defeat of Democratic moderates like Senators Mark Begich (D-AK), Kay Hagan (D-NC), Mary Landrieu (D-LA), and Mark Pryor (D-AR) has had very little effect thus far in moving the Democratic mean leftward. To some degree, their exit may be counterbalanced by the retirements of liberal Democrats like Senators Tom Harkin (D-IA) and Carl Levin (D-MI).

The second set of images shows the percentage of Democratic and Republican legislators in both chambers with DW-NOMINATE scores less than -0.5 or greater than 0.5, making them more ideologically extreme. There has been little change in these values between the 113th and 114th Congresses, with the exception of a marked increase in the proportion of House Republicans with scores greater than 0.5.

These image are from a new stand-alone DW-NOMINATE that can be run daily as new roll calls are cast. We will have more to say about this software at a later date.

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An Update on the Presidential Square Wave

Below we plot the first dimension DW-NOMINATE Common Space scores of the presidents in the post-war period, which we refer to as the “presidential square wave” due to its shape. An ideological score is estimated for each president throughout the entirety of their tenure in office by scaling their “votes” on a subset of roll call on which they announce a position (measured using CQ Presidential Support Votes). Negative DW-NOMINATE scores indicate greater liberalism and positive scores indicate greater conservatism. The presidential scores are directly comparable across time and with members of Congress.

These presidential DW-NOMINATE scores are estimated using all available CQ presidential support roll calls through 2013. CQ does not issue all of its presidential support roll calls until the print version of its congressional roll call guide comes out, and so only a fraction of the 2014 votes are available.

Very little has changed from the last presidential square wave. President Obama fits the spatial model estimated by DW-NOMINATE extremely well, with over 95% of his “votes” correctly classified. Obama has moved very slightly leftward (-0.367) and is now just to the left of LBJ (-0.346) and right of Truman (-0.368), though this trio is virtually ideologically indistinguishable. President Eisenhower is the most moderate president (0.293) of the post-war era.

Among members of the 113th Congress, President Obama is very ideologically close to Representatives Stephen Lynch (D-MA) [-0.364] and Loretta Sanchez (D-CA) [-0.369] in the House, and Senators Patty Murray (D-WA) [-0.367] and Mark Udall (D-CO) [-0.359] in the Senate.

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House and Senate Polarization 1879 – 2014

Below we show the polarization of the Political Parties for the 1879 through 2014 period (46th to 113th Congresses). Polarization is measured by the distance between the means of the Democrat and Republican Parties on the first (Liberal vs. Conservative) DW-NOMINATE dimension. Polarization is now at a Post-Reconstruction high in both the House and Senate.

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Below are the Party means for the House on the first DW-NOMINATE dimension. For the Democrats we show the means for the Northern and Southern wings of the Party (We use the CQ definition of South; the 11 States of the Confederacy plus Kentucky and Oklahoma). In the past three Congresses the difference between the Northern and Southern Democrats has disappeared.

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Below are the corresponding Party means for the Senate on the first DW-NOMINATE dimension. The pattern is essentially the same as the House. However, the Southern Democratic Senators as a group tend to be more moderate than their Northern counterparts.

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Below are the Party mean graphs for the House and second for the second DW-NOMINATE dimension. This dimension usually picks up regional differences between the two major parties. Before the Civil War the second dimension picked up the North vs. South division on Slavery. In the Post Reconstruction period the second dimension picked up regional differences on soft vs. hard Money (bimetalism, gold and silver) and beginning in the late 1930s Civil Rights. In the past 20 years the second dimension has faded in importance and issues that used to divide the parties internally — e.g., gun control, abortion — now load almost entirely on the first dimension. This is explored in detail in Keith T. Poole and Howard Rosenthal (2007) Ideology and Congress. Note that in the figures below the parties have almost converged on the second DW-NOMINATE dimension. Voting in Congress is almost entirely one-dimensional. The first dimension now accounts for over 93-94 percent of the roll call votes.

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