Himself.
At this point re-election is his to lose.
If you want a sports analogy, it is the fourth quarter of a football game and as the quarterback of a team up by two touchdowns and a field goal and the ball on the opponents' 30-yard line as the two-minute warning approaches he must avoid throwing an interception or handing the ball to a runner who has a tendency to fumble.
In that situation, the QB will take a knee to run out the clock! Don't you know anything about football?!
Of course I know the usual strategy -- you try to use the clock so that the other side has no chance at getting 17 points. But there might be people on this Forum who know nothing about American football. I could talk about the nickel defense that allows the other team to score a touchdown, but in a time that the team behind can't get away with.
In essence, the election of 2012 is for President Obama to lose, and not for someone else to win. Vanity and folly can appear in unlikely places, and we all know what can defeat President Obama.