Below are graphs of the House and Senate means from 1879 through 10 July 2015. We computed the means from our Weekly CS DW-NOMINATE Scores.
In a previous post we showed that polarization is asymmetric and due to the Republican Party moving sharply to the Right after 1976. Several people sent us e-mails asking if this was due to the realignment of the South (the 11 States of the Confederacy plus Kentucky and Oklahoma [the definition used by Congressional Quarterly]) into the Republican Party. The answer is No. In the House and Senate graphs below we show Northern and Southern Republicans along with Northern and Southern Democrats. In the House the Northern and Southern Republicans moved in tandem during the entire post WWII period:
In the Senate we only show Southern Republicans after 1961 because there were so few Republicans from the South before then. The Southern Republican Senators are more Conservative than the Northerners but beginning in the mid to late 1990s the two wings begin to converge. In contrast, the remaining Southern Democratic Senators are a bit more Conservative than their Northern counterparts.
In terms of Party Polarization it is at a Post-Reconstruction high in both Chambers. The House Republicans are slightly less Conservative in the 115th but the House Democrats are slightly more Liberal. Hence, this produced a small uptick in polarization (so far) in the 115th House. In the Senate the increase in polarization is due to both Parties moving away from the Center. However, it bears repeating, overall, the increase in polarization in both chambers is primarily due to the Republican Party moving to the Right.