I have despaired more and more recently watching Hillary and Barack (whom I admire greatly) snipe away at each other. Hillary cannot win the delegate or popular count even if she wins overwhelmingly in PA and some other primaries yet to be held. That means the nomination is in the hands of the Super Delegates. If they don't decide things decisively in June after Puerto Rico, I predict the convention will deadlock and go to multiple ballots. Barack will argue he has the most votes and most delegates; Hillary will argue that Barack can't get elected (and that may be largely true if race continues to be an issue and the Clinton camp seems determined to continue to make it one.)
The only solution if the Democrats really want to guarantee victory in the fall is if they settle on a compromise candidate: Imagine a Gore/Obama ticket. Unbeatable.