Dean is the first canidate who I have ever seen, who is hostile to centerists and moderates. People that are in the middle of the road, do not dislike George W. Bush, they do have doubts about his policies though, and are wishy washy in his decisions, they do however seem to personally like him.
Dean is living in a different reality along with his base, they just assume, that unless your a right wing extremist, you hate Bush. If you do not hate Bush personally, then your a right wing radical and all the insults that come with it. This is not the way to get the moderate voters, and portraying yourself as an extremist (as dean is choosing to do for some odd reason) is not going to get him elected, and as the DNC is concerned, may cost the party big time in the general election in regards to congressional elections.