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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