Dean is a bit more subtle than you give him credit for.Torie, don't insult my intelligence.
DEAN: ... which doesn't -- no, I don't believe that. I can't imagine the president of the United States doing that. But we don't know, and it'd be a nice thing to know.
Now which is it? If he can't imagine a POTUS doing that, what is it he doens't know?
Same difference. Your quote is his damage control effort. You would like even less what he originally said. He just wanted to float the idea out there, without saying he knew it was true. He did not flat out accuse Bush; Dean is too smart for that. JMO.