Bart Stupak has decided to retire.
Article in part

Michigan Rep. Bart Stupak, the congressman who led anti-abortion rights Democrats in the House during health care negotiations, will retire this year, CBS News has learned. He is expected to announce his plans later today.

Had Stupak sought re-election, he would have faced challengers from both the left and the right backed by interest groups angered by Stupak's health care vote.

That prompted the conservative Tea Party Express to launch a $250,000 ad campaign against Stupak this week. The group also scheduled a handful of stops on its bus tour in Stupak's district. "
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Why is he retiring?