Incumbents

(Updated below)

John Sides has a nice post up demonstrating that, contrary to some earlier speculation, incumbents still won most (86%) of the House elections in which they competed.  If this is his main point, I agree:

What is the bottom line? Don't read too much into a few isolated early primaries (see: Robert Bennett) or into polls where vast majorities of Americans disapprove of Congress or even indicate their willingness to vote for the challenger. 

I would throw in a little perspective, however.  It's absolutely correct that in House elections, the bottom line is that incumbents win.  Why they win, however, is another story.  There are two major factors that determine House elections.  One is, in fact, the evaluations votes have of the candidates, and for that incumbents do have an advantage, and from all accounts maintained that this year.

The most important factor, however, has nothing at all to do with candidates; it has to do with party.  Party is the most important factor in House voting, and since most House districts have lopsided advantages for one party, most House elections are essentially determined long before specific candidates are chosen.

That's easiest to see in the two districts in which GOP incumbents lost this year, one in Louisiana and one in Hawaii.  In both cases, Republican victories had been flukes (a crook on the ballot for the Dems in one, and a split Democratic party running two candidates in a special election in the other), and once the Dems got themselves a (single) acceptable candidate, that was the end of the story. 

So, yes, ignore the polling that says that Congress is unpopular, or that people want incumbents to lose.  But when John says "Incumbents still dominate congressional elections," I'm going to disagree with his wording.  Parties dominate congressional elections.  Incumbency is an advantage -- yes, even in 2010 -- but party is the biggest story.

UPDATE:  See Andrew Gelman's post at the Monkey Cage, in which he estimates the current size of the incumbent advantages at about 6% in House general elections, and notes that it doesn't seem to have moved much this year.  I fully agree with his final thought: "Once you're in, you generally get to run again and represent your party in the general election. This is the most important way in which incumbents "dominate": they don't need to compete for the nomination on a level playing field each time."