The 112th Congress closed unceremoniously this month with a series of votes (by the House and Senate) to avert the “fiscal cliff”. With this data, we can now analyze roll call voting in the 112th Congress in its entirety and place the amount of Congressional polarization seen over the last two years in historical context. In a series of plots below, we show that partisan polarization has dramatically increased in the 112th Congress in both chambers. And, as has we have previously discussed, this phenomenon has been asymmetric: contemporary polarization of the parties is almost entirely due to the movement of congressional Republicans to the right. Polarization is measured as the difference between the Republican and Democratic means on the first DW-NOMINATE dimension, which represents the ideological (liberal-conservative) scale.
In the first plot, we show our primary measure of polarization – the difference in party means – illustrating its evolution between 1879 and the present. As can be seen, the ideological distance between the parties in Congress grew to record levels between the 111th and 112th Congresses. By this measure, polarization in the 112th House (1.069) has doubled from the 96th House (1979-1981, 0.528), and risen over 50% from the 102nd House (1991-1993, 0.676). Polarization in the Senate – which has remained the more moderate of the two chambers – has surpassed its previous high in the 46th Senate (1879-1881, 0.832), with polarization in the 112th Senate jumping to 0.845. It is now safe to say that polarization in Congress has reached an all-time high, exceeding even levels seen during the late nineteenth century.
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Next, we isolate the locations of the Republican and Democratic means over time in each chamber. Both parties have continued their drift away from the ideological center in the 112th Congress: The Democrats to a greater extent in the House, and the Republicans to a greater extent in the Senate. This is not entirely surprising, since most of the Democratic casualties of the 2010 midterm elections were moderate-conservative, Southern “Blue Dog” Democrats, which has produced a more liberal Democratic caucus in the House. The 112th House also included many freshman Tea Party-affiliated members, but the Republican caucus was already very conservative after its ranks dwindled following losses in the 2006 and 2008 Democratic wave elections. However, the Senate Republican caucus was more strongly affected by the 2010 elections, which sent a cadre of very conservative Republicans to the 112th Senate. This group includes Senators Marco Rubio (R-FL, with a first-dimension DW-NOMINATE score of 0.619), Rand Paul (R-KY, 1.000), Pat Toomey (R-PA, 0.654), Mike Lee (R-UT, 0.999), and Ron Johnson (R-WI, 0.721).
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Finally, we provide a more detailed look at the previous figures by plotting the location of the 10th and 90th ideological percentiles of the parties over time. That is, arraying the party members from most to least-moderate, we plot the first dimension DW-NOMINATE scores of those in the 10th and 90th percentiles. This gives an idea of the ideological dispersion of the party coalitions. The findings support the previous analysis: in the House, the leftward shift of the Democrats is due more to the movement of the coalition of moderate Democrats to the left (since many of the most centrist Democrats lost in 2010). The location of the most conservative Republicans in the House continued to move away from the center, but the election of some moderate House Republicans (e.g., Reps. Jon Runyan (R-NJ, 0.386), Patrick Meehan (R-NC, 0.356), and Lou Barletta (R-PA, 0.381)) moved the 10th percentile slightly back towards the center. In the Senate, the Democratic caucus was virtually unchanged between the 111th and 112th Congresses (or, for that matter, throughout the 1990s and 2000s). Conversely, the 10th and 90th percentile scores of the Senate Republicans both moved in a conservative directions – particularly the locations of the 10th percentile Republican. For a variety of reasons, a number of moderate Republicans in the 111th Senate did not return to the 112th Senate (e.g., Sens. Bob Bennett (R-UT), Arlen Specter (R-PA), George Voinovich (R-OH), and Mel Martinez/George LeMieux (R-FL)).
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We have previously written about asymmetric polarization, arguing that the primary driver of contemporary partisan polarization has been the steady movement of congressional Republicans to the right. This trend appears to have continued through the 112th congress. House Republicans – despite a large majority earned in the 2010 midterm elections – have continued their rightward drift, adding more conservative members than moderate members. Senate Republicans also became a more conservative group in the 112th Congress, while Senate Democrats remained mostly ideologically static. Some of this phenomenon is attributable to the fact that Democrats – particularly northern Democrats – were already holding liberal policy positions in the 1960s. The “Great Society” programs enacted during the 1960s have appeared to represent the leftward edge of what is practically achievable in American public policy (for example, from an ideological standpoint, “Obamacare” is not more liberal than Medicare, enacted in 1965). Congressional Democrats have staked out this position and have mostly maintained it in recent American history. Congressional Republicans, on the other hand, continue to pioneer new ideological territory along on the rightward edge of American public policy. It remains unclear whether and how long this pattern can persist.