Via David Frum, I see that liberals are turning against Jon Tester, one of the darlings of the netroots during the 2006 campaign, for some of the votes he’s had to cast to remain viable in Montana. Politico reports:

“Not only will I do absolutely nothing to help his reelection bid, but I will take every opportunity I get to remind people that he is so morally bankrupt that he’ll try to score political points off the backs of innocent kids who want to go to college or serve their country in the military,” wrote Daily Kos founder Markos Moulitsas, who fired the opening shot on his highly trafficked blog.

“To me he is the Blanche Lincoln of 2012 – the Democrat I will most be happy to see go down in defeat. And he will. Nothing guarantees a Republican victory more than trying to pretend to be one of them.”

“I think he’s alienated his base in the progressive Democrat circles and he’s going to definitely have a lot of explaining to do,” said Montana Democratic National Committeewoman Jean Lemire Dahlman, who dubbed his vote against DREAM “a grave mistake.” “I was surprised. It didn’t seem to jive with what I expect of him.”

Tester may be the most vulnerable Democratic incumbent in 2012. He probably wouldn’t have won the first time around if Republicans had nominated someone other than Conrad Burns. But Tester’s troubles with the left may explain why Jim Webb has been so reluctant to move his voting record appreciably to the right.