Is Obama unstoppable?

[Mehmentum]

Any incumbent with an approval below 50% constantly before the election is never unstoppable.

This.  We're not talking about some formidable juggernaut of modern politics here.  The only reason you can even call Obama favored is because the Republican field is so dismal.

Not really, incumbents have a whole set of advantages that makes them automatically favored (though not guarenteed) to win.