From a FiveThirtyEight post on presidential approval ratings and re-election chances, a nifty chart:
“What does this mean for Barack Obama?” Nate Silver asks rhetorically, then answers:
Right now, we’re still in the period where the most useful number for estimating his re-election chances is not his approval rating but rather the historical track record of incumbent presidents. … [S]ince the Civil War, 73 percent of incumbent presidents who sought another term won, as have 70 percent since World War II.” Obama’s current approval ratings suggest “a 65 percent likelihood of re-election — but again, this is a really rough guess, based mostly on the high historical batting average for incumbents rather than anything to do with Mr. Obama himself.
FWIW, Intrade is just slightly less bullish on Obama, giving him roughly a 60% chance.