Below we continue our analysis of how the first year of the 112th Congress (2011) fits into the contemporary trend of political polarization. In this post, we use Common Space DW-NOMINATE scores– a measure which permits direct comparison of the ideological (liberal-conservative) positions of members of the House and Senate across different Congresses. Because ideal points are estimated via joint scaling, Common Space scores are constant– that is, each member retains the same location throughout her congressional career (see Poole : “Changing Minds? Not in Congress!”).
Consequently, the differences in the histogram below–which compares the distribution of members of Congress along the ideological (liberal-conservative) scale in the 111th (top panel) and the 112th (bottom panel) Congresses–are from replacement stemming from the 2010 midterm elections. Note the loss of members along the right (moderate) edge of the Democratic distribution, many of whom were centrist Democrats defeated in the 2010 Republican wave. However, many of their Republican replacements in the 112th Congress have proven to be reliable conservative, adding to the peak of the Republican distribution center just to the left of 0.5. We also add the locations of (Senators) Obama and Clinton and Sen. John McCain (R-AZ) for reference. McCain is now clearly in the moderate camp of the Republican Party; while Obama and Clinton, a bit to the left of the center of the Democratic distribution in 2009-2010, are now just to the right of the median congressional Democrat in the 112th Congress.
We also replicate the plot in our previous post that shows the difference in party means in the House and Senate over time. Using Common Space scores supports our finding that there has been a considerable uptick in polarization in the first year of the 112th Congress, continuing a trend which has continued unabated over recent decades.