WASHINGTON--EVERYBODY IS FOR health care. So Andrew Stern, president of the Service Employees International Union, wanted to raise the bar for Democrats.

His challenge: Any presidential contender seeking to win the endorsement of his 1.5 million-member union, the largest in the AFL-CIO, must devise a plan to provide health insurance for all Americans and identify funds to pay for it.

Four of the nine Democratic hopefuls have tried to meet that challenge, with SEIU members featured prominently as some of the candidates announced their proposals. An estimated 41 million Americans are uninsured, and universal health care has emerged as a dominant political issue for the first time since President Clinton's failed attempt in 1994.

That's the kind of clout that some unions command from a crowded field that so far lacks a clear, heads-above-the-rest candidate.

Backing is crucial from SEIU and the 1.3 million-member American Federation of State, County and Municipal Employees, the two largest, fastest-growing and politically active unions in the AFL-CIO.

An overall AFL-CIO endorsement requires agreement among affiliate unions representing two-thirds of the 13 million rank-and-file members, and many labor leaders say that's unlikely. Only two candidates have ever secured such an endorsement: Walter Mondale in 1984 and Al Gore in 2000.

For 2004, only Rep. Dick Gephardt of Missouri, a longtime labor ally, is likely to join that exclusive club, union leaders said.

"Maybe he can do it. I think he's the only one who can do it," said AFSCME President Gerald McEntee, who also serves as the AFL-CIO's political chairman. "But I'm still very skeptical that anybody can do it."

Top union leaders will meet in Chicago Aug. 5-6 for the AFL-CIO's executive council meeting, where labor's political leanings will become clearer. A candidates' forum also is in the works. An endorsement, if it happens, would occur in the fall, McEntee said.

Many industrial and trades unions have been enthusiastic about Gephardt, who opposes free trade policies that have helped shrink their memberships.

The growing service sector unions, led by SEIU and AFSCME, have been more coy about their interests. Both Stern and McEntee say no candidate has emerged as a favorite. They both give credit to Gephardt for being first with an ambitious health care plan.

"He was first one out with something very bold, very comprehensive. He set the mark," Stern said. "I feel like he's re-inflated his candidacy."

About 755,000 of SEIU's 1.5 million members are health care workers. The union also represents many low-wage and immigrant workers, such as janitors, who can't afford health insurance and whose employers don't provide it.

"The issue of health care has, I think, become so far an issue that is distinguishing the candidates, and the hopeful nominee from George Bush," Stern said.

His union has posted ads in airports in New Hampshire and Iowa that read, "Running for president? Don't forget health care." Members will start "shadowing" candidates campaigning in those states, making sure they get asked about health care at every public event.

For AFSCME, which has about 20 percent of its 1.3 million members in health care, the issue ranks high. The Democrats' plans contain carrots to appeal to the bulk of AFSCME's membership: state and local government workers concerned about rising budget deficits.

Some offer aid to help states obtain better rates on prescription drugs, or funds to reimburse state and local governments for providing health insurance coverage to their employees.

Besides the people without any coverage, "you've got 61 to 62 million people who have - to use the words of George Bush - a little itty bitty plan that doesn't get you very far," McEntee said, a reference to President Bush's initial complaint about the $350 billion tax bill that he signed into law Wednesday.

Democrats see health care not only as a defining issue of the 2004 campaign but a means to score political points against Bush, whose health care policies have centered solely on talk of revamping Medicare to provide prescription drugs for seniors.

But they do face political risks. Voters aren't likely to digest all the differences in the plans, and to finance the proposals, the Democrats call for rolling back Bush's tax cuts.

Gephardt's plan would cost $247 billion and would give a refundable tax credit to companies, requiring them to offer health insurance. Former Vermont Gov. Howard Dean's $88 billion package would broaden coverage by expanding existing government programs.

Massachusetts Sen. John Kerry's $80 billion plan also would expand existing programs to extend coverage, and Ohio Rep. Dennis Kucinich wants to create a government-run, single-payer program.