He can win for losing
The polls have closed in Pennsylvania and the vote-counting machines are whirring. Barack Obama is expected to lose, but the margin of the loss is the story. As Dan Balz puts it (quoting how someone else put it) the two candidates "are like publicly-traded companies that need to hit an earnings target to lift their stock price."
If Obama loses by 6-10 points, nothing really changes. If he loses by 10 or more, Clinton will have an opening to argue that the dynamics of the race have changed. If he loses by less than 6, superdelegates are going to start sliding into the O column.
CNN says the race is too close to call right now, in part because of a massive turnout. That should be to Obama's advantage, but who the hell knows. Ed Rendell might be stuffing boxes full of Clinton ballots for all we know.