Obama wanted the weaker challenger - Romney.
Santorum would not have been able to debate Obama - he'd be trailing by 5-6 points right now. Gingrich would have defeated Obama in debates, but I think his favorability numbers would be so low that it wouldn't be near enough for him to catch up.
Romney was the best candidate in the field - he was the only candidate who could at least somewhat please the base while also attract enough moderates. Huntsman would have been great with moderates, but the base would have been furious.
