Note: Raw data and additional graphs tracking congressional polarization can be found at: http://voteview.com/political_polarization.asp.
Now that 2013 — the first year of the 113th Congress — has wrapped up and all 640 roll call votes in the House and 291 roll call votes in the Senate have been run through DW-NOMINATE, we can begin to provide some updates on political polarization in Congress.
The primary measure of congressional polarization — the distance between the party means on the liberal-conservative dimension in each chamber — has continued to set new record post-Reconstruction highs over the last few Congresses, and thus far the 113th Congress appears is no exception to this trend. By this measure (shown in the plot below), polarization increased slightly in the House but more drastically in the Senate. This is likely due to the exit of moderates like Senators Richard Lugar (R-IN) and Ben Nelson (D-NE) and the entrance of non-centrists like Senators Ted Cruz (R-TX) and Elizabeth Warren (D-MA) in the 113th Senate.
Polarization in the Senate continues to lag somewhat behind that in the House, but the two series are very highly intertwined (correlated at r = 0.89). This casts serious doubt on the popular explanation that rising polarization is attributable to the redistricting (or “gerrymandering”) process, since only House districts are affected by this process and of course not state boundaries (see also McCarty, Poole, and Rosenthal, “Does Gerrymandering Cause Polarization?”).
We next plot the party means themselves in each chamber over time. In both the House and the Senate, the Republican Party mean continued to shift rightward on the liberal-conservative dimension. On the other side, the Democratic Party mean actually shifted just slightly towards the center in the House, while moving leftward in the Senate.
In addition, while Southern Democrats as a group remain considerably more moderate than non-Southern Democrats in the Senate, regional ideological differences among House Democrats have shrunk considerably, especially during the 112th House after the wave elections of 2010, which wiped out dozens of moderate-to-conservative Blue Dog Democrats in the South. This group has thus far fared somewhat better in the Senate, but the 2014 elections may not be so kind to Senators Kay Hagan (D-NC), Mary Landrieu (D-LA), Mark Pryor (D-AK), or even Mark Warner (D-VA).