Up till now, Obama's general election argument has been that "the democratic party can only win by offering a clear contrast with John Mccain." Now that the race is sure to go on, I think he should shift this a little.
Let Hillary have the "democrats need to" argument and Obama can take the "America needs to" argument. A contrast with Mccain and a real debate about Iraq, foreign policy, and the economy are not just good for the democratic party, they are necessary in order for the country to move forward. Are we going to hedge our bets and continue the Bush policies or will we change direction? We have to make a decision and in order to do that the stakes need to be clearly outlined and a clear choice must be offered.
America needs this debate. Let that be the rallying cry going forward. Nominating Hillary would simply be kicking the can further down the road and preventing us from addressing the raw issues. He needs to become the issue candidate. Not the policy candidate, the issue candidate.
Another way to go would be to declare himself the responsibility candidate (although that would be a little similar to Mccain's theme tonight). He could call for the media and both campaigns to think about how the debate is affecting the general populace and whether it's a good thing to stir division, racial strife, and suspicion. He could call out gamesmanship, as other diarists have said, and demnad that it stop, not for his sake but for the people's sake. He could say that the candidates have a responsibility to concentrate on more than just winning.
Normally this kind of tact would amount to political suicide but I think that the delegate lead is big enough that it won't hurt him. And it certainly could help him in November.