With all of the 2012 Senate races called, we can now offer a more concrete assessment of the ideological distribution of the 113th Senate, which will convene in January 2013. We previously endeavored to forecast polarization in the 113th Senate based on the likely outcomes of these races. As in that post, we use Common Space DW-NOMINATE scores (which allow for cross-comparability between members of the House and Senate) since new Senators have frequently served in the House. For new members of the Senate who have not compiled a congressional voting record that we can scale, we impute scores by first regressing the scores of members of the 112th Senate onto their state and personal characteristics. We then use the coefficient estimates to predict the scores of Senate candidates who have not served in Congress (e.g., Elizabeth Warren and Ted Cruz). The results of the regression analysis can be found here.
Below we plot the distribution of the first dimension (representing liberal-conservative position) Common Space DW-NOMINATE scores of members of the 112th Senate (in the top plot) and the 113th Senate (in the bottom plot). Our findings indicate that the 113th Senate will be more polarized than the 112th (we measure polarization as the difference in the mean ideological positions of the two parties). In the 113th Senate, the mean Common Space DW-NOMINATE score in each party will be further from the center: from -0.328 to -0.341 for the Democrats, and from 0.445 to 0.485 for the Republicans. Most clear is that the ideological center continues to hollow out in the 113th Senate. This is a product of the defeat (as with Sen. Scott Brown (R-MA)) or retirement (as with Sens. Ben Nelson (D-NE) and Olympia Snowe (R-ME)) of moderate Senators. In addition, several Senators (Sens. Kay Bailey Hutchison (R-TX), Herb Kohl (D-MI), and Jon Kyl (R-AZ)) who were near the mean of their parties will be replaced by Senators-elect who are solidly in the liberal or conservative wings of their parties (Ted Cruz (R-TX) and Reps. Tammy Baldwin (D-WI) and Jeff Flake (R-AZ)).
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Note: We treat Senator-elect Angus King (I-ME) as caucusing with the Democratic Party, something he is widely expected to do.