Below we use Weekly Common Space DW-NOMINATE scores to plot the means of the House and Senate Democrats and Republicans 1879 – 2015 (May 31). The Democrats in the House for the first time since 1921 are homogeneous across the North and South (the 11 States of the Confederacy plus Kentucky and Oklahoma). The Republicans in the 114th House have almost exactly the same mean as they had in the 113th House so we may, at last, see a leveling off of polarization. Indeed, the slight increase in polarization in the House from the 113th to the 114th (see below) is due to a slight shift to the left of the Democratic Party mean due to the Southern Democrats shifting left.
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In the Senate the Southern Democrats are more moderate than the Northern Democrats but both groups have shifted to the left. The Republicans have shifted to the right. This is why the uptick in polarization in the Senate is larger than the House (see below).
The results indicate that polarization (i.e., the distance between the two parties on the first dimension [Liberal vs. Conservative]) has increased slightly in both the House and Senate. In an earlier post we thought that the polarization in the Senate had dropped a little. That was an error. We used the wrong dataset. The figure below is correct and uses all the roll calls through 31 May 2015.
I bears repeating that the trend to polarization has been largely due to the Republican Party moving to the right since the 1976 elections. In contrast, the Northern Democrats in both Chambers have barely budged in the past 40 years. What has happened is the the Democratic party has become much more homogeneous.