BAYH TO RETIRE…. A huge surprise out of Indiana this morning, which will make the 2010 midterms that much more challenging for the Democratic majority.

Indiana Sen. Evan Bayh will not seek re-election this year, a decision that hands Republicans a prime pickup opportunity in the middle of the country.

“After all these years, my passion for service to my fellow citizens is undiminished, but my desire to do so by serving in Congress has waned,” Bayh will say.

Like Sen. Byron Dorgan’s (D-N.D.) announcement in January, Bayh’s retirement comes completely out of the blue. The incumbent Hoosier had already raised $13 million for his re-election campaign, and while Republicans thought Bayh might be vulnerable this year, recent polls showed him with seemingly insurmountable leads over his GOP challengers. Indeed, less than a year ago, the National Republican Senatorial Committee conceded that it fully expected Bayh to run and win a third term.

Bayh’s retirement gives Republicans yet another pick-up opportunity in a traditionally “red” state, but it’s probably a stretch to call it a GOP lock, at least at this point. The Republican field is, after all, led by Dan Coats — an inside-the-Beltway corporate lobbyist who hasn’t lived in Indiana for a decade.

As for the possible Democratic field, it’s hard to speculate who might consider a campaign — there have been no rumors since no one seemed to have any idea that Bayh was even considering retiring. That said, Indiana’s congressional delegation does include five Democrats: Reps. Peter Visclosky, Joe Donnelly, Andre Carson, Brad Ellsworth, and Baron Hill.

It will also be interesting to see if Bayh changes his approach now that he need not worry about re-election. Bayh has, after all, been an exasperating “moderate” at times. Will this decision free him up to be a more reliable vote on key issues? Time will tell.

And for those keeping score, there are now six Republican Senate incumbents who have decided not to seek re-election, and three Senate Dems. Expect the media to characterize this as a mass Democratic exodus.

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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.