Here is a summary of Clinton supports who want to vote for McCain.
"Obama has said a lot of stuff, but nothing with a concrete vision," says Book. "I have heard at least some sensible things from Senator McCain. He's a comeback kind of guy and I've heard from many other people who are clearly leaning in his direction.
In any bitterly-fought contest like this year's Democratic primary, there are bound to be disaffected voters on the losing side, says Costas Panagopoulos, professor of political science at Fordham University.
But he predicts that party defections will be much more common this year, especially to the detriment of Democrats and Obama due to McCain's moderate reputation.