The nation's largest single union announced it is holding off announcing its endorsement decision for a Democratic presidential candidate until Wednesday even though the union's members voted for their preferred candidate on Thursday.

President Andy Stern said the Service Employees International Union (search) acceded to a request from the American Federation of State, County, Municipal Employees (search) to hold off on announcing its endorsement until the AFSCME executive board meets Wednesday and also decides who they will endorse.

The two unions will then make a joint announcement.

Most quarters had expected SEIU, which has 1.6 million members, to back Howard Dean (search), and his campaign told Fox News that the vote Thursday was in fact for the former Vermont governor.

Dean was at the SEIU member meeting on Thursday, and Stern said he was excited about the decision.

"We have reached a decision and we are hopeful that there are other unions who share our members excitement for Dr. Dean's candidacy," Stern said as Dean stood alongside him smiling.

Top union officials told at least three Democratic campaigns Wednesday night that Dean was getting the SEIU endorsement.

Dean is a major contender for AFSCME's endorsement. John Kerry (search) and Wesley Clark (search) also are being considered, though if SEIU did endorse Dean and plans a joint announcement with AFSCME, their chances may be minimal.

AFSCME, with 1.5 million members, is the SEIU's biggest rival. It had planned to make its announcement in December but moved up after SEIU decided to act. The two have put down their differences when they realized they needed to unite if they had any hope of defeating President Bush in 2004.

AFSCME's endorsement is considered the Holy Grail for Democrats because the union spends more money on elections than any other. President Gerald 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.

SEIU and AFSCME initially overlooked Dean as a marginal, quirky candidate from a small state. But that changed as he surged in fund-raising and state polls in key states such as New Hampshire and Iowa and began attracting large, boisterous crowds.

In an interview with Associated Press radio, Dean said an endorsement from SEIU would be "a really big boost" to his campaign, citing the union's size and diversity.

The endorsement is a blow to Dick Gephardt (search), who has staked his second try for the White House on the support of organized labor. In his 27 years in the House, the Missouri congressman has carried labor's banner on trade legislation and other issues. But for SEIU and other large service unions, defeating Bush in next year's election trumps loyalty.

Backing from SEIU, the largest union under the umbrella of the AFL-CIO, provides Dean with thousands of crucial ground troops in early primary states and helps him diversify his campaign. The union represents janitors, health care workers and other service employees, many of whom are minorities and women.

SEIU has mapped an aggressive, intensive voter mobilization effort for 2004 that includes making 7 million phone calls, distributing 6 million fliers, visiting 10 million homes and running six mobile action centers in converted tractor-trailers.

Union officials also plan for 500,000 members to donate to the effort, totaling $20 million.

SEIU is the largest union in the early primary state of New Hampshire, with 7,500 members. It also has a large presence in other early primary states, including Michigan, with 45,000; Wisconsin with 10,000; and Washington, with 50,000.

In delegate-rich states such as California and New York, SEIU has 530,000 and 350,000 members, respectively.

Fox News' Ellen Uchimiya and The Associated Press contributed to this report