Neither Obama nor Clinton will run as an independent, because it would be idiotic. Whether or not you believe they care about the welfare of the United States or the Democratic Party (I think they care about both, but others are more cynical than I), they definitely care about their own political careers, and running as an independent would be political suicide. They'd be casting away all the Democratic Party connections they've worked so hard to build.

Such a gambit could only be justified by actually winning the White House. But the only way that can happen is if the independent candidate pulls significant support from the Republican side, offsetting the effect of the split among Democrats. Neither Obama nor Clinton can possibly hope to draw that much support away from McCain, and they know it.

Running as an independent would only guarantee a McCain victory. Say what you will of the Democratic contenders, neither wants to be the next Ralph Nader.