Although Dean wasn't actually all that different from Kerry, as a governor rather than a senator, he was a much more forceful executive personality when it came to the issues (health care, grassroots funding to fight lobbyists, civil unions, anti-war). It would be an even more intense election with a bigger online dimension from the Internet organizing model that Dean pioneered, but he alienated the DLC Democrats with the "Democratic wing of the Democratic Party" talk and that would hurt him. The Rovian model of putting gay marriage on the ballot to juice rural conservative turnout in states like Ohio would still be used here, of course. He does worse than Kerry in the suburbs but just barely picks up Iowa with his more populist style. Probably still picks Edwards as his running mate- North-South pairings were still orthodoxy to that generation of Democrats, and Edwards had a broad appeal.
President George Bush (R-TX) / Vice President Dick Cheney (R-WY) ✓
Governor Howard Dean (D-VT) / Senator John Edwards (D-NC)