Measuring Polarization through Chamber and Party Medians over Time (1879-2012)

Are congressional Republicans, as a recent New York Times article suggests, “stampeding to the right”? Below we use first dimension DW-NOMINATE scores, which measure the locations of members of Congress along the ideological (liberal-conservative) scale, to plot the median location of the overall chamber (House and Senate) and the Democratic and Republican coalitions in each chamber over time. The median is a useful measure of polarization because, unlike the mean, it does not cancel out two extremes (in this case, two ideologically distant parties).

Both figures indicate that the median Republican and Democrat has moved towards their respective ideological poles in the contemporary period, but this effect is considerably larger in the House than the Senate and for Republicans more than Democrats. Because of this and the fact that partisan control of the House has become more volatile in the recent past (prior to 1994, Democrats controlled the House for forty years), the chamber median in the House has become much more volatile. This is important because the median member of the House can be thought of as “pivotal“: the difference between a winning 218-member coalition or a losing 217-member coalition. As a consequence of two polarized parties–neither of which are likely to assemble long-term governing coalitions–policy swings are more frequent and dramatic.

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