"Serious primary challenges to incumbent U.S. senators aren’t that common, but they aren’t unheard of, either. Since 1980, four sitting senators have been defeated in primaries, and several more have faced political near-death experiences in them.
Generally, it’s easy to categorize these primary challenges. There are two basic varieties: ideological, with an exercised party base seeking retribution for an incumbent’s dissent from the party line; and pragmatic, with party members responding to the perceived electoral deficiencies of the incumbent.
And then there’s the Democratic Senate primary now under way in Arkansas, which seems to be a perfect hybrid of these two types. With the latest poll showing Blanche Lincoln’s challenger, Bill Halter, within 13 points of her, that primary -- now just seven weeks away -- has become the hottest Democratic contest in the nation.
Lincoln, who’s in the final year of her second term, has managed to pull off a somewhat remarkable feat, infuriating both the left-of-center base of her party and her state’s right-leaning general election audience at the same time. Thus, the challenge she’s receiving from Halter doesn’t neatly fit into either of the above categories."
more:
http://www.salon.com/news/feature/2010/03/30/blanche_lincoln_primary_challenge_halter