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Leading into 2012, there was talk of a primary challenge to President Obama - primarily from the party left. Bernie Sanders was one of the names to bring up the idea, although he likely wouldn't have taken the plunge himself given he had reelection that year.
Howard Dean was one name floated. Dennis Kucinich perhaps?
Mike Gravel talked about it, but I doubt he'd get too far.
Jim Webb opted against going for reelection in 2012. Might he have tried?