For McCain, obviously, the stakes are higher. He has got to do something bold and attention-grabbing in order to gain some momentum in this race. But even if he does well, it might not be enough. He has to win big tonight. He has to be Ronald Reagan in 1980 - or, perhaps more appropriately, Bill Clinton at the town hall debate in 1992. Short of that, he may not be able to change the underlying dynamics of this race. And if he clearly loses tonight, well, fughgeddaboutit.
In the end, though, I'll bet that both these guys bring their A-game tonight. They know what the stakes are. I expect that, when this thing is over, the pundits will once again essentially declare it to be a draw, which would make it a win (or a hold) for Obama by default. I don't think Obama really has to do all that much right in order to win tonight; it's just that McCain has to do SO much, that I find the chances of that unlikely.
But hey, this is the 2008 election, so what the hell do I know?