The power of incumbency is revered in politics simply because, unlike so many other superstitions, it's legit. Incumbents win reelection to federal office far, far more often than not. Plus about 19 more "far"s.
In fact, data compiled by Bloomberg News' Greg Giroux (and available here) indicates that only 155 incumbents have lost reelection since 1968. Which sounds like a lot until you consider that there have been almost 11,000 contested races over that time (biannual House races in every seat and the full Senate every six years). So we're talking about about 1.4 percent of the time, incumbents lose. If we exclude incumbent-versus-incumbent matches thanks to redistricting, the number drops to 124 incumbent losses or just over one percent of all races.
Which is one reason that Rep. Kerry Bentivolio's (R-Mich.) loss on Tuesday night is exceptional. But the other is that Bentivolio lost badly. As of writing, the not-yet-final returns have him down 66.3 to 33.7 percent to challenger Dave Trott. That's a spread of 33 points, once you round up. To put this in context, the average margin of defeat for an incumbent since 1968 was 13.3 percent.
Bentivolio outdid that margin handily. His was the eighth-worst margin of defeat for an incumbent since 1968, according to Giroux's data. If you include incumbent-versus-incumbent matches, it drops to ninth (excluding the odd Bella Abzug/William Ryan race of 1972).
The winners of those overwhelming incumbent ousters, by the way, include several names you're likely familiar with. California's Marty Martinez lost to Hilda Solis, who became President Obama's first Labor Secretary. Bob Inglis lost to Rep. Trey Gowdy, who's made a name for himself as head of the House select committee investigating the Benghazi attack. The all-time biggest margin title is held by Max Baucus, who is now the U.S. ambassador to China. And Buz Lukens lost to an Ohioan named John Boehner.
As you might expect, states with more House seats tend to see more incumbent ousters. But looking only at the 124 non-redistricting incumbent losses, there are perhaps some surprises on where they've occurred. Like Georgia, which is something of a hotbed of incumbent losses. And New Jersey, which ... isn't.
Bentivolio is the sixth Michigander to lose his seat. The closest races that incumbents have lost in that state were by six points, including in 1992, when Guy Vander Jagt lost to an upstart named Pete Hoekstra. Hoekstra is also in the news today.