A recent New York Times Magazine article examined ideological differences between the progressive and centrist wings of the Democratic Party, using the potential Democratic Senatorial primary race in Maryland between Reps. Donna Edwards and Chris Van Hollen as an example of “the great Democratic crack-up.”

Below we use DW-NOMINATE scores to illustrate the ideological positions of Democrats and Republicans in the current House and Senate (Note: these scores are from a new stand-alone Common Space DW-NOMINATE program that can be run weekly as new roll calls are cast; data available here.)

In the present (114th) Congress, it is clear that Republicans occupy a wider swath of ideological territory than Democrats. It is easy to identify distinct ideological clusters of Republicans, but not for Democrats (a point we make here). The distance between, for instance, Republicans Senators Lisa Murkowski and Ted Cruz dwarfs that between Democratic Senator Elizabeth Warren and former Senator Hillary Clinton. In fact, the distance between Democratic Representatives Donna Edwards and Chris Van Hollen is more comparable with the distances between Senators Lisa Murkowski and John McCain or between Senator McCain and Representative Paul Ryan.

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The images below show trends in the ideological dispersion of the parties in Congress since 1879 by plotting the variance in DW-NOMINATE scores among House and Senate Democrats and Republicans in each Congress. Higher values indicate greater ideological dispersion within the parties in both chambers.

From a peak in the mid-twentieth century (when the Democratic Party was split between its Southern and non-Southern wings), the ideological variance among House and Senate Democrats has steadily declined. Congressional Democrats are now more ideologically unified than at any point since the early 1900s. The ideological variance of Congressional Republicans has been flatter over the last century, but there is some suggestion of a jump in dispersion among Senate Republicans in the 112th Congress (following the 2010 midterm elections).

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