The point is not that every incumbent is vulnerable — certainly Chuck Schumer isn’t — but that every incumbent might be.

The threat might come, as it did to congressional Republicans in 2006 and 2008, from Democrats taking advantage of voter exhaustion with the GOP in districts and states that should have been safe conservative territory. Or it might come, as it has over the past month, from within the incumbent’s party itself.

If Bob Bennett can be ousted in Utah and Alan Mollohan can be ousted in West Virginia — in their own party’s primaries in seats they not only they held for decades, but that their fathers held in turn before them, in states noted for the relative docility of their party’s internal electorate — then anything can happen.

In this respect, Joe Lieberman’s 2006 loss to Ned Lamont in the Connecticut Democratic Senate primary was not just a result of leftist disaffection with Lieberman’s pro-war voting record. It was an early indication of a profound change in American politics made possible by a confluence of events — the technological revolution in fund-raising and attention-gathering made possible by the Internet and the greater degree of passion and participation in politics by voters who are driven more by ideology than by partisanship.

That revolution is only going to be enhanced by the Supreme Court’s determination earlier this year that much of the architecture of campaign-finance “reform” — which is designed almost explicitly to limit the ability of outsiders to target incumbents — is unconstitutional.