To add on to Atrios here, the case of Rep. Ellen Tauscher is interesting. Here's a centrist Democrat who wrested a swing seat from a rock-ribbed Republican. A couple years later, though, her seat was redistricted, becoming overwhelmingly Democratic. Her own vote totals show the shift. In 1996, she wins with 49%. In 1998 and 2000, she takes 53% of the vote. In 2002, however, she's redistricted and gets 76%. In 2004, she gets 66%.
In American Congressional politics, incumbency is a huge advantage. And, as Atrios says, "[primary challenges] provide one of the very few checks on legislator behavior that the people have." Now, you could argue that Tauscher was accurately reflecting her district even a few years ago, but there's a perfectly fair case to be made that, since the redistricting, she's too conservative. Hell, she's made it: She bitterly opposed that particular redistricting for the precise reason that it would expose her record to more liberal voters. So why should a respect for incumbency overwhelm the actual opinions of her constituents? Why shouldn't, just as a democratic matter, there be enthusiasm for a decently-funded, more liberal challenger who can claim to better represent the district?
