Study: Republican primaries polarize. Democratic primaries don’t.

Are moderates more likely to lose primaries? Not particularly, say the political scientists at VoteView. The 19 incumbents who have lost primary challenges since 2006 — 11 Republicans and 8 Democrats, in case you’re wondering — were slightly more moderate than their counterparts in Congress. But not much.

More interesting, however, is the voting records of their replacements. Six of them couldn’t be assessed because they lost in the general election. But of the five challengers who defeated incumbent Democrats and went on to serve in Congress, two were more liberal their predecessor, two were more conservative, and one voted pretty much the same. Of the eight Republican challengers who won the election, six proved more conservative than their predecessor, and only two were more liberal.

In other words, Republican primaries served to push the party to the right, while Democratic primaries didn’t. And remember: These numbers don’t include conservative primary challenges that helped convince Republican moderates to drop out, like the one Olympia Snowe faced in Maine, or that pushed the incumbent to switch parties, like the one faced by Arlen Specter in Pennsylvania or Lisa Murkowski in Alaska, or primary challenges that were unexpectedly close and so scared the incumbent into voting in a more conservative fashion both before and after the race.

(VoteView) The Republican primaries are, in other words, sending a clear message to Republican incumbents about what they must do to protect themselves from future primary challenges. In that way, one defeated incumbent can change the voting patterns of three dozen incumbents who think they could be vulnerable in the next election.

“This provides a partial explanation for polarizing trends in Congress,” writes the VoteView team. As you can see in their graph, which I’ve posted above, Republicans have moved much further to the right than Democrats have moved to the left, and there’s been a sharp spike in Republican polarization over the past few years, which is the period of time in which these primaries have been happening.