The thousands of union members who will march through Downtown Pittsburgh today symbolize a movement eager to reassert its strength at the polling place as well as in the workplace.

Unions' share of the work force has been declining for decades, and the presidential season presents an opportunity for labor groups to change that.

The most obvious way is by supporting candidates sympathetic to their agenda, ones who will push laws, for example, that make organizing easier.

But the other part of a chicken-or-the-egg equation is that politicians are more likely to support organized labor's issues if unions show they can turn out voters and that there is a political price to be paid for ignoring their agenda.

As with almost everything else surrounding a hyper-accelerated nominating season, the efforts to court unions began early.

Last week, Sen. Hillary Rodham Clinton won the support of the International Association of Machinists and Aerospace Workers along with that of the United Transportation Union. In a surprise, Sen. Christopher Dodd, D-Conn., picked up the endorsement of the International Association of Firefighters, and former North Carolina senator John Edwards, who is poised to receive the endorsement of the United Steelworkers today, won the backing of the United Brotherhood of Carpenters and Joiners.

While there is debate on the power of endorsements, the efforts of the candidates suggest they still matter. It seems as if would-be presidential candidates have been courting union leaders since the last election ended.

If anything, those early results suggest there is no consensus "labor candidate," in the Democratic field.

"The firefighters are important, the carpenters are important, but my sense is that there is not one candidate who is going to get the lion's share of the endorsements," said Gordon Fischer, a former chairman of the Iowa Democratic Party. "In that sense, unions, like Democratic voters themselves, will be split. That could blunt the impact of unions."

Different story in 2004

The possibility of scattered union allegiances is a contrast to the picture at a similar stage of the last Democratic nomination fight, noted Carrie Giddins, communications director of the Iowa Democratic Party.

In that contest, former Rep. Richard Gephardt, of Missouri, was by far the leader in labor endorsements, with the backing of major forces including the USW and Iowa locals of the United Autoworkers. Later in the winter, preceding the Iowa caucuses, two major unions, the American Federation of State, County and Municipal Employees, and the Service Employees International Union endorsed the insurgent candidacy of Howard Dean.

But the winner on caucus night was Sen. John F. Kerry, who had a small but intense labor cadre drawn from the same Firefighters union that endorsed Mr. Dodd last week.

The lesson of that experience is open to debate. It conferred bragging rights on the IAFF as well as on its candidate, but Mr. Kerry, though once a distinct long shot, had other things going for him in the final weeks before the balloting. He received a much-noted endorsement from Christie Vilsack, the state's popular First Lady, just as the steam was going out of the Dean steamroller. That establishment imprimatur helped give a sense of possibility to the revived Kerry campaign. And days before the caucuses, a long-lost crew mate emerged with the tale of the Massachusetts senator's heroism in saving his life in Vietnam.

Still, without the organizational heft of the IAFF, it might have been Mr. Edwards, who finished a relatively close second in Iowa, rather than Mr. Kerry, who would have surfed the wave of caucus momentum all the way to the nomination.

"If labor is sort of all over the lot, it could neutralize some of the things labor wants to do," said Philip Dine, a veteran labor journalist who is the author of the forthcoming book, "The State of the Unions."

"On the other hand, it provides the opportunity for an energetic and smart union to really have a disproportionate impact."

Mr. Dine suggests that is what the firefighters achieved in Iowa four years ago.

"The firefighters just put together this plan, they based it on who they were as a union and the nature of the state, and it was effective ... the SEIU, AFSCME sent in all these people from out of state and it didn't mean as much," he said.

The caucus process, which traditionally attracts a smaller turnout of voters who must give up several hours to attend a meeting rather than just pop in an out of a voting booth, is believed to place a heightened premium on organization.

"Unions have built-in organizations. They have lists," noted Mr. Fischer. "If you want to set up a phone bank, unions can do that."

"Caucuses are about putting people on the ground," said Ms. Giddins. "It's about having bodies for activities; going door-to-door. Labor does that. That's very important out here."

AFSCME and the UAW have the largest memberships in Iowa. The Steelworkers union and Communications Workers of America also have significant influence. And while the primary calendar is in flux, the official Democratic schedule still calls for Nevada, another caucus state, to host the second nomination round.

Bruce Raynor, president of UniteHelp, a major force in the hotel and apparel industries, told the Wall Street Journal last week that he supports Mr. Edwards, although Amanda Cooper, a UniteHelp communications official said later that it was a personal, rather than union, endorsement.

In either case, it suggests a significant potential advantage for Mr. Edwards in Nevada, whose hospitality industry provides the core of the state's union strength. The former senator, who has been a supporter of organizing drives among hotel and hospital workers, stands to have special appeal to unions such as UniteHelp and SEIU.

Linda Merfeld, president of the Finley Hospital unit of an SEIU local in Dubuque, Iowa has discussed her union's organizing battle with the hospital with Mr. Edwards. It's a measure of the disproportionate attention her state attracts that she's had conversations about the local labor battle with Mrs. Clinton and Sen. Barack Obama as well.

Targeting NAFTA

Mr. Edwards also made a point of identifying with issues of interest to the USW and other industrial unions before the spotlight of campaign coverage was at full candle power. Chuck Rocha, political director of the USW, pointed out earlier this summer that Mr. Edward had been a supporter of the union in its battles with the Goodyear Tire & Rubber Co.

Mr. Edwards has similarly been increasingly critical of trade agreements, notably the North American Free Trade Agreement, that have been the targets of years of bitter criticism from industrial union officials. In recent speeches, Mr. Edwards, who was not in Congress when NAFTA passed, has cited the pact as evidence of the failings of "presidents from both parties," in a clear indictment of one aspect of the record of Mrs. Clinton's husband. Former President Bill Clinton presided over the passage of both NAFTA and the extension of permanent normal trade status with China.

"The trade policies of President Bush have devastated towns and communities all across America. But let's be clear about something," Mr. Edwards said in a speech earlier this summer. "This isn't just his doing. For far too long, presidents from both parties have entered into trade agreements, agreements like NAFTA, promising that they would create millions of new jobs and enrich communities. Instead, too many of these agreements have cost us jobs and devastated many of our towns."

Still, just as there's no consensus labor candidate in the Democratic field, there are no labor skeptics either. The candidates' lifetime AFL-CIO voting records, according to a compilation by FactCheck.org, cover a relatively narrow range between 97 percent, for Mr. Edwards and Rep Dennis Kucinich, of Ohio, and 85 percent for Sen. Joe Biden of Delaware. Mr. Obama was right behind the top scorers, at 96 percent, with Mrs. Clinton at 93 percent and Mr. Dodd at 91 percent. All the candidates in the field have close ties to labor organizations in his or her own state.

Ms. Merfeld, the SEIU organizer, supported Mr. Edwards four years ago, although her union had endorsed Mr. Dean. This year, she's undecided.

"I'm not leaning toward anyone right now,'' she said. "Luckily, from a union standpoint, the Democrats have awesome candidates to choose from.''