My coauthor and friend Andrew Gelman, in the course of plugging my previous post about incoming Republican moderates and liberals in the 112th Congress, asks me:

There’s only one thing I wonder about. Even if everything Boris writes is correct–and I have no reason to doubt him–he’s still only coming up with 10 moderate Republicans, out of a total of 200 or so. That’s not a lot.

Well, I found two more (Debicella in CT-4 and Bruun in OR-5), so it’s now 12. Of course, not all of them will get elected, so only a fraction of those will get a chance to be the fightin’ moderates of the Republican Class of 2010.

Congress has been getting more ideologically polarized since the 1950s, with a dramatic rise since the mid-1970s. This means that Democrats are becoming ever more liberal, and Republicans are becoming ever more conservative. In the 110th Congress, both the House and the Senate are more polarized than they have ever been (since the Civil War and Reconstruction). The 111th was more polarized still. Here’s Nolan McCarty’s plot of Congressional polarization:


I predict the 112th will be the most polarized yet. That’s not saying much; the trend has been very strong. Or in other words, the story of increasingly conservative Republicans is “dog bites man.” However, in a wave election, quite a few liberal districts and/or liberal candidates from the winning side will get through. In 2010, that’ll be liberal Republicans; in 2006 and 2008, it was conservative Democrats. That’s “man bites dog,” and therefore newsworthy in its own right (or so I think as an amateur journalist).

Nolan McCarty, Keith Poole, and Howard Rosenthal, in a 2009 AJPS paper identify the advance of polarization by two paths: 1) districts electing representatives more like them (eg, liberal districts electing liberals and conservative districts election conservatives), and 2) for a given district, Democrats becoming more liberal, and Republicans more conservative. The former effect is called sorting and the latter effect is called divergence. Here’s a freely accessible but older version of the paper; the final version is gated here.

Of course, the irony is that these moderate and liberal Republicans will be uniquely vulnerable in the post-wave elections of 2012 and beyond. Djou and Cao, for example, got in under extraordinary circumstance; so will some of the Fightin’ Liberal 12 I identified. That’s the sorting effect which has been cutting down on the number of liberal Republicans and conservative Democrats in the past few decades.

But once they’re defeated, they’ll be replaced by considerably more liberal Democrats. Until then, they are considerably more conservative than the Democrats they have or will have defeated. This is the divergence effect.

Thus, Djou is undoubtedly a liberal Republican in HI-1. But Colleen Hanabusa is much, much more liberal, as shown in her voting record as a Hawaii state senator. If she attains office, she’d be as liberal as Senator Patty Murray (WA) or Senator Patrick Leahy (VT). Thus, conservatives have to face the fact that the alternative to electing a liberal Republican is often electing an even more liberal Democrat.