Honestly, I think it's really unlikely. Momentum alone is usually enough to keep the incumbent in office unless [a] things are really awful or [b] the challenger presents an extremely compelling alternative. (Which is generally *why* the batch of candidates in off years are so bad - the ones whom the party thinks actually have a chance are advised to wait for the next term so as not to be seen as already defeated.) While things are far from great, Obama's scored enough coups in foreign policy (and kept things from getting much worse at home) that I imagine he can win against a dude who frankly doesn't seem to understand how to connect with human beings.