The Achilles heel of both Clinton and Obama is their perceived inability to be Commander in Chief. Obama just doesn't have the experience. Clinton is a woman, which the Repubs will try to twist into unsuitableness for a War President, and she has more experience but not that sort. The Repubs were even able to paint John Kerry as unable to handle a war portfolio, and he's a decorated war hero.
If the Republican candidate is John McCain - war veteran, former PoW, hawk - I don't see how Clinton or Obama can win. Perhaps I'm just feeling pessimistic today.