Former US Vice President Al Gore - the defeated Democratic candidate in 2000 - says he will support Howard Dean's campaign for the White House.
Dean (left) endorsed Gore in the presidential race in 2000
The endorsement of such a mainstream figure is a major boost for Mr Dean.
The former Vermont governor is already the Democrats' frontrunner, but has been accused of appealing only to the left of the party.
The successful candidate will run against George W Bush in the presidential election due next year.
"Howard Dean really is the only candidate who has been able to inspire at the grassroots level all over this country the kind of passion and enthusiasm for democracy and change and transformation of America that we need in this country," Mr Gore told a rally in New York.
"We need to remake the Democratic Party, we need to remake America, we need to take it back on behalf of the people in this country."
The BBC's Justin Webb in Washington says Mr Gore's endorsement will push Mr Dean even further into the lead.
He says Mr Dean's problem has been that his supporters are often portrayed as zealots who are talented at using the internet to raise money but lack political maturity.
The backing of Mr Gore - who narrowly lost the last presidential election in controversial circumstances to Republican Mr Bush - is seen as giving much-needed solidity to Mr Dean's campaign.
"Dean's been knighted by the ultimate insider. It's game, set and match. It's over," Democratic consultant Dean Strother told the Associated Press news agency.
Even before the news of Mr Gore's endorsement, polls had suggested that Mr Dean was likely to win the early electoral tests among democratic voters in Iowa and New Hampshire.
Connecticut Senator Joe Lieberman, Mr Gore's running mate from the 2000 campaign who is also seeking the Democratic nomination, said he was surprised by Mr Gore's announcement.
He added that it was now "less likely" that there would be a place for Mr Gore in a Lieberman administration.