The Clinton Dillema
I think Josh may be on to something here.
The conventional wisdom on why Clinton is running so hard against Obama is that she has to damage him so much that superdelegates will quail at nominating him and will throw their weight behind her. And if she can't get it that way then she will destroy Obama out of spite. Thus the image of Hillary "I'll-do-anything-to-win" Clinton candidacy is born.
But what if, as Josh's analysis suggests, Clinton is running this hard because it is simply a requirement of keeping a viable (albeit it on life-support) candidacy going through the next couple of months. After all, if she backs off, her candidacy will truly spiral into irrelevancy (money will dry up, media attention will move on, etc.) She has to run hard, not to destroy Obama, but simply to keep herself in the game.
But then the question becomes: why even bother? If her position is so tenuous, why not simply call it quits before you really embarrass yourself? Could it simply be blind-naked ambition that keeps Clinton going?
Or could it be that she actually believes that she is the better candidate. Could she honestly believe that it's to dangerous to put forward an untested candidate as the nominee? If you were in her position and believed that Obama was really the weaker choice between the two of you then wouldn't you stick it out when nearly 50% of the voters continue to support you?
No one who runs for President does so from a position of humility (if it looks like they are then they are just really good at faking it). None of them would be in the race if they didn't honestly believe they were the best choice. You don't persuade a politician to cut their future short simply by throwing numbers at them. You have to approach them with the understanding that dropping out of a race is one of the hardest things for a politician to do (ask Al Gore or Howard Dean).
I think democrats who want this race to be over are taking exactly the wrong approach by simply calling on Clinton to take the high road. If you are going to get her to drop out you have to convince her that Obama would be a good choice for nominee. If you can pierce her perception that he is simply not a viable alternative than you can get her to start doubting her own position. And once that happens then she is that much more likely to withdraw gracefully.
Or you could just accuse her of being some Machiavallian shrew who would prefer to destroy the party if she doesn't get to make the rules. It's clear that that strategy has worked wonders so far.
(I'd like to note that it is once again surrogates who are aggravating this situation. Obama himself has said that Clinton should leave only when she wants to leave. It's his fan base that is shoring up her backbone to keep the race going.)