Ross Douthat contemplates Obama's options in the event he does not win the nomination:

If Hillary loses the general election, Obama presumably becomes the media's go-to-Democrat, and thus the face of the party, in a McCain or Romney Administration, and he would be far and away the front-runner, I would imagine, for the nomination in 2012. 

If Hillary wins, on the other hand, he'll be in a position not unlike John McCain vis-a-vis George W. Bush after 2000, except he'll be younger, more charismatic, and possessed of a much larger and more devoted core of supporters in his own party than McCain has ever enjoyed in the GOP.

Which is to say, he'll be able to play the above-the-fray, trans-partisan figure whose support President Hillary needs but can't take for granted - the role McCain played for much of the Bush Administration - without having to do nearly so much of the base-shoring-up spadework McCain's been forced to attempt in his quest to win the '08 nomination.

How about he just wins the nomination now, and saves us from four more years of Republican rule?