Dean has criticised the Clintonian centre-right of his party – "Washington Democrats" - for neglecting the party's more liberal and leftwing base.
In turn, Democrat rivals have claimed Dean is naïve and unelectable. Many Republicans agreed and hoped Bush would end up facing Dean.
That contest once looked a distinct possibility, thanks to Dean's unrivalled use of internet campaigning, strong fund-raising and the backing of Democrat heavyweights such as former Vice President Al Gore.
Born in East Hampton, New York state, Howard Brush Dean graduated first from Yale in 1971 and then at Albert Einstein College of Medicine. He opened his own medical practice in 1981 with his wife Judith in the state of Vermont, where he lives today.
Dean was elected to the Vermont state assembly in 1982, became lieutenant governor in 1986, and then governor in 1991 (when the incumbent suddenly died). He was re-elected to that post five times, but stood down in 2002 to prepare his presidential challenge.
Religion: Congregationalist (wife and children Jewish)
Spouse: Judith Steinberg Dean
Current post: None
Military record: None. Dean was excused service from the Vietnam war on medical grounds
Thanks to his campaign to create a universal health care system in Vermont (defeated in the state legislature in 1993), Dean became known as one of the country's most liberal governors. He did succeed in extending free health care to 92% of his constituents, including 96% of the state's children.
He also signed into law a bill allowing Vermont's homosexual couples to acquire the legal equivalent of marriage.
But the social liberal dubbed himself a "fiscal conservative", resisted increasing government spending and managed to balance the state budget every year of his governorship.
The former doctor's banner policy is to increase health insurance coverage, mainly by merging existing programmes. He says he can achieve this and keep the annual cost below $100 billion by repealing the Bush tax cut of 2001. Dean arguably remains the candidate most vocal about balancing budgets.
Former Vice Presdent Al Gore (L)
has given Dean his blessing
Less liberal-sounding is his plan to increase security spending – though as well as funding enhanced airport and border surveillance this would include more cash to raise an army of first aid responders plus programmes to limit nuclear proliferation.
But Dean has criticised the Patriot Act and how its implementation has eroded civil liberties. He has accused the Bush administration of misusing security legislation to mistreat its Arab and Muslim citizens through racial profiling etc.
Dean has successfully captured the imagination of many liberal and peace-loving Americans with his opposition to the Iraq war - arguably the most familiar stance of any Democrat candidate.
He insists there was no clear evidence Iraq possessed nuclear or biological weapons, or posed any immediate threat. He says Bush should have worked more closely with the United Nations and US allies to disarm or depose Saddam Hussein.
Dean is no pacifist, however, and says he supported every US military operation between Vietnam and Iraq. He also argues the massive defence budget should not be cut while the US faces the risk of "terrorist attacks".
Although Dean says Israel needs to give up many of its illegal settlements on occupied Palestinian land to achieve peace, he remains hawkishly pro-Israel. He has repeatedly said the major issue in the conflict is "Palestinian terrorism", not the Israeli occupation, and backs Israel's policy of assassinating resistance leaders.
What supporters say
His campaign offers America new hope, his supporters are breathing fresh air into the lungs of our democracy... showing a way to escape the grip of big money and to confront the shame of forgetting those in need."
- Bill Bradley, former New Jersey senator
What critics say
"We need more than simple answers and the latest slip of the tongue."
- John Kerry, Massachusetts senator and Democrat rival
Dean has been criticised for being error prone, such as his comment in April 2003 that "We won't always have the strongest military" - although many neutral observers would see such remarks as statements of the obvious. Rivals also accuse him of weakness in foreign policy.