Messina, Baucus's former chief of staff and campaign manager, now heads the new pro-Obama group Organizing for Action. He said he would "absolutely" do what's needed to reelect Baucus.

The six-term senator's approval ratings have recovered a bit since the health care battles of 2010, but he still looks vulnerable to a challenge. Two Republicans are in the race already, though neither has high name recognition at this point.