A first point, which should hardly need to be made, is that this race is close. Clinton and Obama have divided the party right down the middle. If you look at that fact alone, what can people calling on Hillary to give up now and drop out be thinking? We know that the pledged-delegate count will not settle this. The outcome will depend on the superdelegate vote, and that is not pre-ordained. Hillary would be mad to surrender in this situation.
Second, especially if Hillary can narrow (or overturn) the various vote-count gaps by August, the candidates' standing in the polls will count too. This is the main thing I would add to Jay's take. Suppose that Obama has a small lead in pledged delegates, and Clinton has forced, in effect, a tie in the popular vote. But also suppose that by then the national head-to-head polls show her with a ten-point lead or better over Obama. That will sway some of the superdelegates--as it damn well ought to, since their most important task is to get a Democrat elected.
A lot can happen between now and the summer. (Despite Obama's fine speech on the subject, it is not yet clear how much harm the Wright affair has done to him. There might very well be more such setbacks, for either side.) By the summer, the polls might be tied too, of course--which would be bad for Hillary, because she has to overturn Obama's pledged-delegate advantage. But they might not be tied--and if they aren't, that will matter.
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