By John Whitesides, Political Correspondent

WASHINGTON (Reuters) - The AFL-CIO's largest union is likely to endorse Democratic presidential contender Howard Dean (news - web sites) next week, political sources said on Thursday, a step that would hand the former Vermont governor one of labor's biggest prizes and deal a blow to rival Richard Gephardt.

Reuters Photo

The 63-member executive board of the 1.6 million member Service Employees International Union, one of the nation's most politically active unions, will meet on Nov. 6 to consider an endorsement.

"It is becoming clear that the passion of the members lies with Governor Dean, and that ultimately the decision before the board will be to either endorse him or endorse no one," said SEIU President Andy Stern, adding that no final decision will be made until next week's meeting.

"Until that meeting takes place, any speculation as to what the result of that vote will be is just that: speculation," he said in a statement.

The union's highly coveted endorsement, which has been courted by all nine Democrats vying for the right to challenge President Bush (news - web sites) in 2004, would be a coup for Dean.

It would enhance his already extensive grass-roots organizing muscle and give him a boost in the early caucus state of Iowa, where he has battled Gephardt for the top spot in the polls, and in the first primary state of New Hampshire, where he leads the polls.

Gephardt, who has strong labor support nationwide and the backing of 20 international unions, has counted on labor to put him over the top in Iowa and elsewhere.

But the Missouri congressman failed to win the backing of the full AFL-CIO labor federation, which declined to make an endorsement this month because no candidate had the required support of AFL-CIO unions representing two-thirds of the membership.

AFL-CIO President John Sweeney freed affiliate unions to make their own endorsement decisions, but other Democratic presidential contenders had been urging the SEIU to hold off.

Stern and other union officials were meeting with retired Gen. Wesley Clark (news - web sites), the latest candidate to seek their support, later on Thursday, a union spokeswoman said.

The Democratic candidates courted the SEIU's most active members at a meeting in Washington last month, and Dean told Stern the union's backing would be crucial to help him shorten the Democratic nominating race.

Dean picked up his first AFL-CIO endorsement, from the painters union, earlier this week, along with one from the California Teachers Association.

If the SEIU decides not to make an endorsement next week, it could free its local unions to make their own decisions, union spokeswoman Sara Howard said.