They hate Clinton viscerally, beyond any sense or reason. They are more frightened of her being president than anything, and want to head her off at the pass, as it were. They will ultimately vote for McCain in the general election, however, no matter who gets the Dem nomination. They just don't want Hillary to get it.
They are also the kind of (white) people I could imagine giving a "Bradley Effect"-type answer to a pollster over the phone. "Yes, we love Obama." But in the voting booth, fuck that, it'll be McCain.
-B.