I highly doubt Dean could beat Bush. One of the reasons Bush prevailed in a close election was because they were able to stick the "Massachusetts liberal" charge on Kerry. If the GOP could make that charge stick on Kerry, a moderate compared to Dean, then they would have no trouble branding Dean as a left-wing curiosity too liberal for America. Also, too, the GOP would raise the "inexperience" charge over Dean, the governor of a state with 3 electoral votes (imagine a GOP twist on Sarah Palin in 2008, ironic isn't it?) Anyway, I think Dean's campaign would've been more energetic than Kerry's, but in the end I think Bush's second term wouldn't have been in doubt.
Remember that Wisconsin, Pennsylvania, and New Hampshire all went for Kerry by 2 points or less, so Bush could've conceivably won there.
Bush/Cheney - 321
Dean/Gephardt - 217
