The Olympia Snowes of U.S. state legislatures aren’t hankering to run for Congress. (Carolyn Kaster — Associated Press)

Imagine that you’re a state legislator thinking about running for Congress. The polarization in Congress is pretty apparent to you. Democratic members are increasingly liberals and Republican members are increasingly conservative. But here’s the problem: you don’t fit that pattern. You’re a moderate. What will you do?

According to new research (gated version here) by political scientist Danielle Thomsen, you’re much less likely to run for Congress. Thomsen argues that in an era of polarized parties, how well politicians “fit” their party ideologically will affect whether they decide to run for higher office.  Drawing on a 1998 survey of state legislators, Thomsen finds that moderate Republicans were less likely than conservative Republicans to believe they could win a House primary and less likely to value a seat. There were fewer difference between moderate and liberal Democrats — which Thomsen attributes to the nature of the Democratic Party at that point in time, which featured a larger number of moderate to conservative Democrats serving in the House.  Of course, many of those Democrats are now gone.

Perhaps more telling, however, is not what state legislators say in a survey but whether they actually do run for Congress.  Drawing on data from 2000-2010, Thomsen finds that moderates in both parties are less likely to run than those who “fit” the party better: liberals in the Democratic Party and conservatives in the Republican Party.  Here’s her graph illustrating this finding, with the ideologies of several members of Congress noted for the sake of illustration:

Since most state legislators don’t run for Congress, the average probability of running is fairly low.  But that probability is still much greater among conservative Republicans and liberal Democrats than among moderates in their party.

Thomsen’s finding suggests that polarization has created a sort of vicious cycle. Greater polarization makes moderates hesitant to run. This means that the candidates who do run tend to be more ideologically doctrinaire.  And when those candidates win, polarization is reinforced or even exacerbated.