Now that the 114th House has conducted 66 roll call votes and the 114th Senate has voted 53 times, we now have enough data to take a very preliminary look at ideological polarization in the new Congress. Following standard practice, we use first dimension DW-NOMINATE scores as measures of legislators’ liberal-conservative positions.

The first two plots below show the mean score of Democrats (Northern and Southern) and Republicans in both chambers over time. Though we are hesitant to put too much stock in these results at this early date, it looks like polarization (the ideological distance between Democrats and Republicans) is on pace to take a slight uptick in the 114th House but may level off in the 114th Senate. In the Senate, this is due to a slight shift in the Republican mean back to the center. Some of this may be due to the exit of Senator Tom Coburn (R-OK) [whose score is 0.792] and the entry of Senators Cory Gardner (R-CO) [0.216] and Mike Rounds (R-SD) [0.398]. There may also be a procedural aspect as Majority Leader Mitch McConnell (R-KY) has allowed several votes on amendments.

Interestingly, the defeat of Democratic moderates like Senators Mark Begich (D-AK), Kay Hagan (D-NC), Mary Landrieu (D-LA), and Mark Pryor (D-AR) has had very little effect thus far in moving the Democratic mean leftward. To some degree, their exit may be counterbalanced by the retirements of liberal Democrats like Senators Tom Harkin (D-IA) and Carl Levin (D-MI).

The second set of images shows the percentage of Democratic and Republican legislators in both chambers with DW-NOMINATE scores less than -0.5 or greater than 0.5, making them more ideologically extreme. There has been little change in these values between the 113th and 114th Congresses, with the exception of a marked increase in the proportion of House Republicans with scores greater than 0.5.

These image are from a new stand-alone DW-NOMINATE that can be run daily as new roll calls are cast. We will have more to say about this software at a later date.

Click images to enlarge