Sarah Palin won her debate with Joe Biden not primarily because she did well (she did), but because the expectations for her were so low going in. The expectations for McCain's performance tonight are so high, it is simply impossible for him to meet them. The talk is that this has to be a knockout blow for the Arizona Senator--this for a politician whose penchant for negotiation and compromise make knockouts a hard thing to achieve.
Even if he does well, it won't be good enough.
Listen for commentators to be saying things like, "This had to be a decisive win for McCain, and clearly he didn't accomplish that."