Who's getting primaried, and why?

Hans wrote two interesting posts recently (here and here) on the subject of asymmetric polarization. That is, he wondered why DINOs (Democrats in name only) aren't singled out for retribution by liberals the way that RINOs (their GOP equivalent) are by conservatives. I share Hans' impression that there's a difference across the parties here, and that this almost certainly has something to do with the fact that Republican officeholders seem to be moving right far more quickly than Democratic officerholders are moving leftward.

I wanted to point out, however, the work of Robert Boatright, who has a book coming out on congressional primaries. As Boatright notes, contrary to much speculation, primaries aren't becoming more common in congressional races, and ideology is only one of many motivations for primarying an incumbent. Primaries, he notes, are most common within a party that is having a good year; the same things that motivated Republicans to challenge Democrats in 2010 also motivated Republicans to challenge sitting Republicans.

Perhaps most interestingly, he finds some important differences between the parties in terms of ideological primary challenges, and not in the expected direction. Below is a scatterplot of Republican House members from 1972 to 2008, plotted by the ideological makeup of their districts and their ideal point on congressional roll calls. Those who were challenged in primaries are highlighted. The important thing to note is that some incumbents who were primaried were too liberal for their districts, but some were too conservative:

Here's the same graph for Democratic House members. Interestingly, basically every primaried Democratic incumbent was too conservative for her district. This makes it look like Democrats are more the party of ideological purity than Republicans are.

Now, this dataset encompasses the 1970s and ends in 2008. It is quite possible that the roles have reversed in recent years, with Republicans embracing ideological purity and Democrats becoming more tolerant of moderate behavior by incumbents. But there seems to be nothing endemic to the Republican Party that would make them more obsessed with the purity of their officeholders.