Cook Partisan Voting Index for U.S. House seats and percentage of seats won by Democrats

Cook Partisan Voting Index for U.S. House seats and percentage of seats won by Democrats

It's amusing to see Sean Trende saying "I blame society," but that's kind of the upshot of his thorough new piece, the latest salvo in the forever-war among political scientists and other elections junkies on the topic of whether gerrymandering is or isn't to blame for the rise in polarization in the nation's body politic. The title is "Gerrymandering isn't to blame for D.C. impasse," but the article is more subtle and even-handed than the headline might suggest.

For starters, Trende acknowledges that gerrymandering is a real factor, and that it helped House Republicans to win more seats than one would expect based on popular vote, as the median district moved further to the right in the latest round of redistricting. However, Trende points out that there may be bigger factors at work, and that they originate with voter behavior, not with the cartographer's pen. For starters, there's the matter of increased geographical self-selection, with likely Democratic voters packing themselves more and more into urban areas; they aren't distributed efficiently, which makes it difficult to draw a Democratic-friendly map.

And perhaps most significantly, there's the problem of the decline in ticket-splitting. No doubt you've noticed how few House Dems are left in districts that Romney won, and vice versa how few GOPers are left in Obama districts, whereas in previous decades it wasn't terribly unusual to find Blue Dogs in red rural districts or (though more rarely) Rockefeller Republicans in blue suburban districts.

Trende demonstrates that gradual change through a variety of interesting tables. The overall number of "Highly Partisan Districts" has increased, while at the same time, the ability of a person from the "wrong" party to get elected in one of the HPDs has dwindled even more dramatically.

Take districts that went for Republican presidential candidates by 20 points more than the national average, for example (so-called "R+20" seats, using the parlance of the Cook Partisan Voting Index): In 1992, 15 percent of those seats were held by Democrats, while today it's only 3 percent. Nine percent of R+15-19 districts were held by Dems in 1992, compared with 2 percent today, while among R+10-14 districts, Dems held 18 percent in 1992 but 2 percent today.

If Democrats were able to win seats across each of these tiers at the same ratio as they did 20 years ago, they would comfortably control the House. Since there are more HPDs that are Republican than Democratic (and because the median seat is now around R+2), you can see how that decline in ticket-splitting hampers Democrats.

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