The Clinton campaign's hopes of winning rest on the possibility that the superdelegates will vote, en masse, for her. This is unlikely, to put it mildly, unless we find out something new about Obama that absolutely shatters his credibility (he names Steps as his favourite band, for instance) - for at least a couple of reasons:
First, they know that, given Obama's lead in pledged delegates, all hell will break loose in the party should they tip the decision to Clinton - and they'll be blamed.
Second, many of them will depend on the kinds of people supporting Obama - young, new voters - to win future elections. These people may walk away from the party if Obama loses. The converse is not true. Clinton's core is made up of loyal Democrats who will vote for Democratic candidates in most situations.
So why are the superdels, or at least the super-duper dels (the Gores and Pelosis) not calling for an end to this race? Why not just get it over with?
PoliSci student Brendan Nyhan has a simple explanation based in economic theories of incentives:
There are two classic economic problems here. The first is that the collective benefits of pushing Hillary out are much larger than the individual benefit to any one party leader (i.e. there's a positive externality). Why would Pelosi or Reid risk becoming a hated figure to millions of Hillary's supporters? As a result, everyone is likely to sit back and hope that someone else will pay the cost of forcing her out.
The second problem is it's difficult to coordinate a joint effort to push her out. In other words, there's a collective action problem. If all the leaders could magically come together to ask her to withdraw, it might be less costly to them individually to push her out, but any effort to make this happen would inevitably leak, generating untold recriminations and infighting. The incentives to defect from such an agreement would also be strong. As a result, no one is likely to chance it.
In other words, it's 'You tell her'...'No - you tell her...'