I don't think he can
If anything, Obama's a mite handicapped by his cool Vulcanoid personality. But that shouldn't prevent him from inserting the knife with calculation and premeditation. I wouldn't be surprised if he does it in the third and final debate. McCain made some crack about being the cool hand at the tiller or some such cliche, well then, it's fair game for Obama to reveal the volcanic Achilles Heel lurking in McCain. Do you want a Vulcan or Klingon's hand on the nuclear trigger?