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.

Click image to enlarge

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.

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

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