The executive board of the American Federation of State, County and Municipal Employees voted unanimously to endorse Dean, and the Service Employees International Union announced it was backing the former Vermont governor.
The endorsements were expected, with news coming last week that AFSCME, 1.5 million-member strong, would coordinate with the 1.6-million member SEIU in making known their preference in the nine-way race.
"We have a candidate who represents our values and who can defeat this president," AFSCME President Gerald McEntee said. "AFSCME is going to mobilize the largest and most aggressive grass-roots campaign this nation has ever seen. Together with Governor Dean, America's working families will take back the White House in 2004."
McEntee formally announced the endorsement at a news conference with Dean and SEIU President Andy Stern.
"SEIU's members are the heart, the soul and the decision-makers of this union," Stern said in a statement. "And as the members learned more and more about the candidates, it became clear that there was an overwhelming enthusiasm among them for Howard Dean."
SEIU and AFSCME initially overlooked Dean as a marginal, quirky candidate from a small state. But that changed as he used the Internet to outpace his rivals in fund raising, and began to attract large, boisterous crowds at events. In particular, he has caught the attention of voters outside the traditional Democratic Party base, making his candidacy attractive to McEntee and Stern.
Dean's union endorsements are a blow to the campaign of Dick Gephardt, the Missouri congressman and former House minority leader who has carried labor's banner in Congress. Gephardt has the backing of some 20 unions, but AFSCME and SEIU were two of the biggest and most coveted prizes.
AFSCME previously had targeted early December for an endorsement but moved up the timetable when the SEIU decided to act this month.
AFSCME's endorsement is considered the holy grail for Democrats because the union spends more money on elections than any other. McEntee was key to Bill Clinton's 1992 campaign success by providing crucial, early support when other unions were backing Sen. Tom Harkin of Iowa.