So what happened? Baucus voted against gun control. And in response, Organizing for Action — the post-campaign incarnation of the Obama campaign — announced a campaign to “punish our enemies.” Note that Baucus was at the top of the enemies’ list:

“Jon Carson, executive director of Organizing for Action, told the Los Angeles Times / Tribune Washington Bureau on Thursday that the group will train its resources against the 45 senators who opposed the legislation, including Democrats Max Baucus of Montana, Mark Begich of Alaska, Heidi Heitkamp of North Dakota and Mark Pryor of Arkansas.” …

This is unlike Specter because Baucus is not changing parties, but it’s similar because polls were showing him very weak back home. Like Specter, he feared a primary (albeit by a more conservative Democrat). The group that’s so close to President Obama that it sells access to him for $500,000 a pop has now driven Baucus out of office for bucking party orthodoxy on gun control last week.