Which Type of Incumbent Is Truly in Danger?

Yes, it stinks to be an incumbent this year, but the truth is 90% of congressional incumbents will probably be reelected. Our system can only tolerate a bum turnover rate of about 10 percent a cycle, even when the "Throw The Bums Out" mentality is driving voter preferences. How is Arlen Specter like Bob Bennett? How is Bob Bennett like Allan Mollohan? How is Blanche Lincoln like Charlie Crist?

Republican pollster Glen Bolger has a theory, and it makes sense to me.

Simply put: "Those who face anger from the grassroots, coupled with a challenger candidate with the resources to get their message out, have challenges."

What he means is that in a hyper-partisan environment, primary voters want purity. They're going to punish people who work with the other side, or who display independence. If the grassroots, be it in the form of the Tea Party movement, social conservatives, or labor unions, or Jane Hamsher at FireDogLake, can find a good candidate to challenge you, you're in trouble.

What counts as apostasy? Depends on the person, the race, and the state.

For Bob Bennett, it was his stimulus package votes, TARP vote, which prompted the Club for Growth to start targeting him. (Later, it was his work with Ron Wyden on a health care bill.) For Trey Grayson, it was merely his connection to the establishment, although if Rand Paul hadn't decided to run, it's hard to imagine Grayson running into the trouble he did. Rand Paul is the exception, not the rule, and using his victory as a way of suggesting that all incumbents are in trouble is misleading.

Charlie Crist's sin was a hug from President Obama and a sense among Republicans that he wasn't one of them at heart.  Arlen Specter willfully changed parties to have a better chance to win, and activists found a better, purer candidate. In Arkansas now, Blanche Lincoln has always had to figure out how to navigate a skeptical electorate, but labor and liberal activists figured out that she was vulnerable, and could be punished for her departures from the party line.

Most Republican incumbents are in "good shape," Bolger writes.

But if they've displayed some independence, if they've cooperated with the administration, AND if a credible, financed challenger can be recruited, then and only then does the anti-incumbent factor truly kick in. Give primary voters a real choice, and when they're frustrated, they'll go for the purest, distilled essence of their ideology.

I'd add a few corollaries.

Proximity to congressional leadership seems to be triggering primary challenges early, as local activists resent the top-down imposition of a candidate. In the case of Harry Reid, it is his status as a leader himself.
Voters don't seem to care that he provides for the state, just as West Virginians didn't seem to care that Al Mollohan was a generous earmarker, or that Blanche Lincoln had secured sugar and ag subsidies. Bragging about bringing home the bacon doesn't work in a year when even Democratic primary voters worry about overspending.