Home > Applying Political Science, Elections > Parties Matter in the Polarization Story

Parties Matter in the Polarization Story

Philip Bump wrote yesterday in the Washington Post’s blog, The Fix, about the relationship between a Representative’s voting behavior and the ideology of the district she represents. To do so, he compares a member’s liberalism (as measured by the National Journal’s annual ranking) with that of the member’s district (as measured by how much more less than average the district supported Romney in the 2012 presidential election–what we political scientists call the normalized presidential vote).

The big take away for Bump is that there seems to be a pretty strong relationship, shown in the graph below. The Pearson’s R for the two measures, in case you were wondering (and you know you were wondering), is 0.834 (p<0.001). Indeed, Bump notes, the districts that everyone expects to flip parties this election are on the wrong side of the y-axis (member voting) given their position on the x-axis (district ideology). The member is either too conservative (e.g., Miller in CA-31) or, more often this election cycle, too liberal (e.g., Matheson in UT-4) for the district.

Source: Philip Bump, The Washington Post, Sept. 29, 2014.

The graph is all well and good for Bump’s purposes, but I think it shows something much more interesting. In order to see what I want to talk about, let’s look only at those districts within +/- 5 points of the average district vote for president–which are shown in the following graph. (Note: I’ve flipped the axes from what Bump had. The liberal-conservative voting index is now on the y-axis and the district vote is now on the x-axis.)


There is no overlap between the two parties here. The most liberal Republican is still more conservative than the most conservative Democrat. Moreover, members representing similar districts (as measured by presidential voting) behave very differently depending on their party affiliation. McCarty, Poole, and Rosenthal (2009; gated) call this effect intra-district divergence. They argue that most of the polarization that we see in Congress today, in fact, is not a function of how districts are drawn but rather how members represent those districts once they are sworn into office. They write, “[T]he secular increase in polarization is not primarily a phenomenon of how voters are sorted into districts. It is mainly the consequence of the different ways Democrats and Republicans … represent the same districts” (678).

This election cycle will continue a long run trend of district sorting as Bump notes, but there is more to the polarization story than electoral sorting. District composition is important, but the various forces pulling members apart–especially the political parties–are more important.

McCarty, Nolan, Keith T. Poole and Howard Rosenthal. 2009. “Does Gerrymandering Cause Polarization?” American Journal of Political Science 53:666-80.

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