Over at The Plank, Josh Patashnik makes an argument about the limited appeal of both Obama and Clinton:

[W]hat's become clear at the end of this primary season is that neither Democratic candidate's appeal is as wide as Democrats would prefer. It's difficult to project what will happen in November from primary results or even general-election polling at this stage, so any such speculation should be taken with a major grain of salt. I think it's fair to say, though, that in general Obama appears to have a problem with working-class whites east of Illinois, and Clinton appears to have a problem with Westerners and more upscale independent-minded voters. This pattern has been remarkably consistent since the beginning of the primary season. My suspicion is that these weaknesses basically cancel each other out, which is why you see both candidates sporting approximately equal-sized small leads over John McCain in national polls.

Those numbers will fade somewhat once the heat of the primary season fades, but I suspect that they'll be more resistant to change among Clinton supporters. Despite McCain's presence as a reasonably attractive GOP candidate, I seriously doubt Obama's coalition of voters would vote for him. On the other hand, Clinton's "hard-working, white Americans" have voted for the GOP in the past and could easily do so in November.

There's been a lot of speculation in the press about why Hillary's staying, in, but this is the only politically viable argument I think she has left. Oddly enough, Saturday Night Live pretty much drove this point home in this sketch: