The 12th ammendment scenario
What if there's a tie in the electoral college?
It's quite possible, polling guru Nate Silver says. The most likely scenario would be that "Obama wins the Kerry states plus Iowa, New Mexico and Colorado, but loses New Hampshire." And then the House of Representatives (with each state's delegation getting one vote) would choose the president, and Obama would win.
Well, maybe. The thing is that if there is a tie, McCain might very well be ahead in the popular vote. In which case, who knows.
Overall, McCain is doing much better in the popular vote than in the electoral college. So there's a chance he will win the popular vote and Obama will win the electoral college (the other way around is much less likely, at least at this point).