Think of the worst possible scenario, an Obama win with a popular vote share of 41%. How does that happen? A third, and maybe fourth party candidate. Two possible entrants along those lines are Donald Trump and New York City mayor Bloomberg.
If either or both of those independently wealthy, and not-really-conservatives run, on the basis that they want to rid the country of a leader that they think is doing a bad job, they'll just ensure that he wins. It's not hard to figure out - a two or three way split of the anti Obama vote only helps Obama. In an electoral college battle that is bound to be close and come down to a few key states, a percentage point or two each is likely to skew heavily towards the anti-incumbent vote. And both potential candidates would probably take mo0re than 1 or 2 percent.
Both candidates are smart enough to realize they are hampering their own stated cause by merely running. So why on earth would they run against him if they want Obama defeated? Because they don't want him defeated, they want him re-elected.