Are Moderate Members of Congress More Likely to be Defeated in Primaries?

With the defeat of Senator Richard Lugar (R-IN) in his primary race against conservative state Treasurer Richard Mourdock, coupled with last month’s losses by moderate Blue Dog Representatives Jason Altmire (D-PA) and Tim Holden (D-PA) in their respective primaries, it is natural to ask about the role of party primaries in the decline of moderates in Congress (see below, where we plot the percentage of moderates – defined as having DW-NOMINATE first dimension scores of between -0.25 and +0.25 – in both chambers):

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To address part of the question of the role of the party primary system in the overall decline of moderates, we focus in this post on whether incumbents who are defeated in the primary are more likely to be ideologically centrists, whether those who defeat them are more ideologically extreme, and if there are differences between the parties (for an innovative study related to this question, see Aaron King, Frank Orlando, and David Sparks’s analysis using the Twitter accounts of primary candidates to estimate their ideological location).

Below we plot the DW-NOMINATE Common Space scores (which allow for cross-comparability between the House and Senate) of the 19 congressional incumbents (11 Republicans and 8 Democrats; 5 Senators and 14 Representatives) who were defeated in their party primary between 2006 and 2012. We also include a rug plot at the bottom of the graph of the first dimension scores of all members of Congress during this period (Democrats in blue ticks, Republicans in red ticks) to show the overall ideological distribution of the parties. For both parties, defeated incumbents are slightly more moderate as a group than their partisan counterparts, though the difference of the mean (ideological) first dimension scores between the two groups is larger among Republicans (0.07, t = 1.22) than Democrats (-0.04, t = -0.87).

What is more dramatic is the difference between successful primary challengers and defeated incumbents. In the second plot below, we use an arrow to show the difference between the DW-NOMINATE Common Space scores of incumbents and the primary challengers who have defeated them (the arrow pointing to the location of the challengers). We lack DW-NOMINATE scores for 6 of these 19 challengers because they were not elected (most of these six ran from the ideological flanks of the party – e.g., Republican Richard Mourdock of Indiana and Ned Lamont of Connecticut – and so data on them would likely strengthen out findings). Of the 5 challengers who defeated Democratic incumbents, 2 were more liberal than the incumbent, 2 more conservative, and for one there is no substantive difference (mean difference = 0.01, t = 0.06). Of the 8 Republican challengers, however, 6 were more conservative than the incumbent and only 2 more liberal, an average shift of 0.20 units to the right (t = 2.23). Though preliminary and based on a small number of cases, these results lend some support the idea that moderate Republicans in Congress are more imperiled in the primary process than are moderate Democratic incumbents (whose ranks have been cut more heavily by retirements and in general elections).

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This provides a partial explanation for polarizing trends in Congress. Below we show the party means on the first (ideological) dimension over time. The Republican mean has moved steadily to the right since the mid-to-late 1970s, and the effects of the primary process are consistent with this trend.

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