In a series of plots below, we show that polarization between the two parties in Congress has continued to increase through 2011 – the first year of the 112th Congress. Congressional polarization is measured below as the difference between the Republican and Democratic means on the first DW-NOMINATE dimension, which represents the ideological (liberal-conservative) scale. With the data from 2011, it has become increasingly clear that Congress is now more polarized than at any time since the late 19th century. Indeed, the growth in the distance between the two parties from the 111th to the 112th House was the largest since that following the Republican takeover of the House in the 1994 midterm elections (0.071 versus 0.085 in 1994):
Next, we isolate the locations of the Republican and Democratic means over time in each chamber. Both parties have been trending away from the ideological middle in recent decades. This pattern is most pronounced for Republicans in the Senate and, especially, the House. Much of this is attributable to the Republican Party’s sharp rightward shift on economic issues: government regulation, taxes, and redistribution. In addition, the dwindling number of conservative, Southern “Blue Dog” Democrats in Congress has produced a more liberal Democratic coalition, with the impending retirements or defeats of Democratic moderates like Senators Ben Nelson (D-NE), Max Baucus (D-MT), and Bill Nelson (D-FL) likely to supplement this trend after the 2012 elections.
Finally, we emphasize the disappearance of moderates in Congress by plotting the percentage of “overlapping” members in each chamber over time. Overlapping members are those Republicans who are to the left (more liberal) than the most conservative Democrat; and conversely, Democrats to the right of the most liberal Republican. At their peak, overlapping members comprised majorities in each chamber. In the last few Congresses, the overlap has vanished; that is, the most liberal Republican is to the right of the most conservative Democrat (in fact, this distance in the House is a sizable 0.187, in the Senate it is only 0.044 – between Senators Susan Collins (R-ME) and Ben Nelson (D-NE)).