This week, Rep. Richard A. Gephardt (D-Mo.) began what backers and opponents agree will make or break his bid for the presidency: a marathon, nine-month struggle to convince skeptical leaders of organized labor that he can win in November 2004.

Labor, stung by Republican victories in both houses of Congress, the White House and in state houses across the country, is determined to pick a winner. Many top union officials remain unconvinced that Gephardt is their key to victory.

Leaders of the AFL-CIO gathered here publicly voiced near-unanimous affection for Gephardt, who has a stronger pro-labor record than any of his competitors for the Democratic presidential nomination. But in private, almost all said they need strong evidence that Gephardt can win votes and raise money to counter his failed 1988 presidential bid and his inability to produce a Democratic majority in the House during his years as Democratic leader.

"We are waiting for him to make some moves. If he does, great; he's our guy, and we'll go with him. If he doesn't, we are going to have to look elsewhere," said a leader of a major industrial union that Gephardt has backed throughout his career, especially in fights over trade legislation.

There was a note of anguish in Gephardt's voice as he sought to win the backing of labor leaders whose causes he has espoused over 20 years.

"I fought for the interests of working families my whole career. These issues are in my bones. My dad was a Teamster, a milk truck driver who told me we had food on the table because he was in a union," Gephardt said after meeting privately with the AFL-CIO executive council. "That is where I am from. Those are the people I grew up with; those are the people I fight for. And I think that will be understood by voters, by unions, by people all over the country."

After Gephardt made his case to the council, Gerald W. McEntee, president of American Federation of State, County and Municipal Employees and chairman of the AFL's political committee, said Gephardt "gave a powerful, passionate speech in terms of where he has stood for workers in the past, where he stands now and where he will stand in the future. . . . He was very effective."

Asked if Gephardt can win, McEntee, who has signaled an interest in the candidacy of Sen. John F. Kerry (D-Mass.), said, "It's so early on, you have to go through the process. We are still quite a long way away from the Iowa caucuses."

More than any other candidate in the contest for the Democratic presidential nomination, Gephardt is banking on labor support, the most important force in party primaries. Of the major Democratic interest groups and voting blocs, including abortion rights supporters, blacks and trial lawyers, Gephardt has been most consistently loyal to labor.

Politics has been labor's major success story of the past decade. It has significantly increased union turnout while other groups have fallen. Despite this success, labor has seen its ally, the Democratic Party, reduced to minority status.

Even more threatening, the ranks of organized labor have failed to grow. Today, the Bureau of Labor Statistics reported that union membership fell by 300,000 from 2001 to 2002, and the size of the AFL, 13.2 million, is only slightly larger than its 12.6 million members in 1955, the year the AFL and CIO merged.

In 1988, labor was crucial to Gephardt's victory in the Iowa caucuses. In 2004, the schedule of caucuses and primaries appears likely to give labor added importance, with such strong union states as Michigan, Illinois, Pennsylvania and Missouri considering advancing their dates to early in the process. In Iowa, labor made up 33 percent of the Democratic caucus turnout, and in these other states, it can produce a fifth or more of the primary electorate.

In 2000, labor demonstrated its power as voters from union families consistently provided higher margins to Al Gore, the endorsed candidate, than nonunion families.

Two-thirds of member unions are required for a candidate endorsement. If the AFL-CIO does not endorse someone, individual unions may choose a candidate to support.

One of Gephardt's friends from his law school days, James R. Hoffa, head of the Teamsters, said: "We are not talking about endorsements; no one is talking endorsements here. All we are doing is gathering information. I know [Gephardt] very well. I have a lot of respect for him. He's a fine man and a great public servant. He has the best record on labor issues, absolutely. I'm not going to make any commitments."

Bruce Raynor, president of the Union of Needletrades, Industrial and Textile Employees, was more blunt: "The number one priority is beating George Bush."

Democratic Sens. Joseph I. Lieberman (Conn.) and John Edwards (N.C.), former Vermont governor Howard Dean and former senator Carol Moseley-Braun (Ill.) also came here (only Gephardt got an invitation) to make the case to labor not only on behalf of their candidacies, but also against making an early endorsement. Only twice, in 1984 and 2000, has the AFL endorsed a candidate before the start of the primaries in a fight for the nomination.

"A lot of these other guys have longer relationships with the unions. My job is to make sure they know me and know who I am. I intend to compete as hard as I can for them," said Edwards as he walked between private meetings.

Lieberman said, "I've had a longtime relationship with a lot of folks in the labor movement, going back to my initial campaigns in Connecticut." He contended that as a moderate, he is best equipped to expand on the number of states Gore won in 2000. "I'm the candidate who can win states outside the blue [Gore] states that we won in 2000."