MSNBC political numbers guru Chuck Todd stated quite definitively on one of the Sunday morning talk shows last weekend that the contest will end on May 6. On the one hand, he's only an analyst, but on the other hand he generally comes off as personally noncommitted and his predictive powers are uncanny.
He predicted that if the NC/IN vote that day becomes a sweep for either candidate, that candidate will be christened the nominee. In the event that it's Clinton, and she has a Pennsylvania win behind her, the argument will be that she "has the momentum"-- that Obama has lost his following the Wright flap and that she's the only electable candidate still standing-- and the Democratic elders will defer to that argument. This prediction factors the generally-held perception that Obama is expected (as of the time of its utterance, last Sunday) to win North Carolina.
Likewise, if it's a sweep for Obama, Clinton's argument will prove untenable, particularly considering the Wright situation which was supposed to have sunk him, and the elders will anoint him then.
So if the vote that day goes as expected according to current polling, and Clinton wins Indiana by a narrow margin while Obama picks up North Carolina, it will be apparent that Obama's "momentum" has not suffered a fatal blow, and that he's still leading respectably in the overall popular vote and delegate count, and that nothing will change that. In this scenario, Obama wins the nomination. On May 6. According to Chuck Todd.
If you acknowledge the merit of Todd's argument-- and I'm inclined to even though I'm skeptical by nature-- then you can already see where this is going. Obama's poll numbers, almost a week later, haven't significantly fallen-- Clinton's have.
So if you're rooting for Obama, get ready to be happy. Five and a half weeks from now.