STUPAK HEADS FOR THE EXITS…. Rep. Bart Stupak (D) of Michigan, who made a name for himself this year by repeatedly threatening to kill health care reform over indirect abortion funding, will announce today that he’s stepping down at the end of his term.

Stupak confirmed his decision to the Associated Press and is expected to formalize it as a press conference at 12:30 pm in Marquette, Michigan.

Sources familiar with Stupak’s thinking describe him as burned out from the long fight over health care in which he emerged as the leading voice of pro-life Democrats wary about the possibility that the legislation would allow federal funds to be spent on abortions.

It’s hard to know whether the “burned out” explanation is the real story. After all, before this week, there was practically no talk about Stupak retiring this year, and the incumbent lawmaker gave no indication that he planned to do anything but seek another term. But right-wing interest groups this week started to go after Stupak with a vengeance, and their attacks appear to have rattled the lawmaker badly.

As we’ve talked about before, not all retirement announcements are created equal — the more significant ones are in districts that are likely to switch party hands. Stupak’s announcement, then, is pretty significant. His district leans to the right, with Bush having carried it twice, though Obama won here with just 50% support in 2008. Expect Republicans to target the district aggressively, an effort they likely wouldn’t have launched had Stupak sought re-election.

Connie Saltonstall, a former local county commissioner, was already taking on Stupak in a Democratic primary, and will reportedly continue to run in his absence. Chances are, however, that party leaders will see Saltonstall as more progressive than the district, and there’s already talk about recruiting Mike Prusi, Michigan’s Senate minority leader, into the race.

For those keeping score, Stupak is the 16th House Democrat to announce his retirement for this cycle, as compared to 18 House Republicans who are also stepping down (19 if you include Florida’s Mario Diaz-Balart, who is retiring from one House seat to run for another).

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Follow Steve on Twitter @stevebenen. Steve Benen is a producer at MSNBC's The Rachel Maddow Show. He was the principal contributor to the Washington Monthly's Political Animal blog from August 2008 until January 2012.