CHICAGO, Illinois (CNN) -- There's a good reason why Democratic candidates court the support of organized labor.
"You often hear people talk about labor in decline, that unions are irrelevant, unions are dinosaurs, that they no longer matter, but in the political arena there's a different story," said Peter Francia, author of "The Future of Organized Labor in American Politics."
"I think organized labor is sometimes underestimated because their share of the work force, the percentage of workers who belong to unions, has dropped off precipitously," Francia said.
Union households make up roughly a quarter of the electorate in most elections. Almost 60 percent of them voted for former Vice President Al Gore in 2000 and for Sen. John Kerry, D-Massachusetts, in 2004.
During the 2006 congressional elections, the AFL-CIO's political program boasted its volunteers knocked on more than 8 million doors and reached out to 30 million voters, many in the union-rich and politically crucial states of Ohio, Pennsylvania and Michigan.
Seven of the 2008 Democratic presidential contenders will gather Tuesday night at Soldier Field in Chicago, Illinois, to try to woo that support. Watch how Democrats are battling for the coveted labor endorsement »
The AFL-CIO-sponsored forum not only will showcase questions from rank-and-file members but also from some of the 12,000 workers expected to attend.
The event is part of the AFL-CIO's 2008 political program. AFL-CIO President John Sweeney says the focus this election cycle is increased emphasis on educating members about the candidates and issues and, of course, fine-tuning the voter mobilization infrastructure that mainly Democratic candidates rely on to get out the vote.
"I think this is going to be our best effort in political activity that the labor movement has ever seen," Sweeney said in an interview Monday.
Workers' issues such as wages and health care are common election themes -- some labor experts say they are even in vogue in the 2008 campaign.
Former Sen. John Edwards, D-North Carolina, called for "smart and safe" trade policies that would put workers' jobs and wages ahead of benefits for corporations in a speech Monday to a union in Cedar Rapids, Iowa.
Sen. Barack Obama, D-Illinois, joined the picket line at Chicago's Congress Hotel, where workers have been striking for more than four years. Obama promised to come back, even as president.
Edwards, Sen. Chris Dodd, D-Connecticut, and New Mexico Gov. Bill Richardson all participated in the Service Employees International Union "Walk a Day in My Shoes" program in which the candidates followed a worker through the rigors of his or her day. Obama will spend Wednesday with a home health care worker and Sens. Hillary Clinton, D-New York, and Joe Biden, D-Delaware, are scheduled to do the program.
Clinton called collective bargaining "absolutely essential to the way America works" in a March speech to the International Association of Fire Fighters.
All of the candidates have traveled to Nevada, an early caucus state, to speak out in support of Culinary Workers Union 226 as it fights for contracts in hotels, restaurants and casinos there.
"We are blessed with an abundance of riches and good candidates who are very strong on so many of the issues that workers have as their priorities," Sweeney said.
Union foot soldiers' support could make or break a candidate in a close election. As a backdrop to Tuesday night's forum, the AFL-CIO's Executive Council is meeting to decide whether or not to endorse a candidate in the primaries.
In 2004, the AFL-CIO could not get the necessary two-thirds vote to support one candidate during the primaries. Several prominent unions split their endorsements mainly between former Vermont Gov. Howard Dean and Rep. Dick Gephardt of Missouri, feeding a perception that labor could not turn out the vote because neither candidate got the party's nomination.
"I think this time the leadership of the labor movement wants to do as an intensive a program as they possibly can on educating and mobilizing workers and want to delay the endorsement as long as they can or until they feel comfortable where their members are at," Sweeney said.
Francia said labor faces some difficult choices.
"John Edwards is certainly, I think, the candidate who is the most front and center on labor issues," he said. "But the dilemma that unions face is whether John Edwards can actually win the nomination because labor does not want to back a loser, and right now Barack Obama and Hillary Clinton are definitely the top two candidates based upon all of the polls. So Edwards is going to have to start making gains, I think, in order to win some of the key union endorsements."
Edwards' "One America" campaign theme is infused with economic populism. His aides touted his trade-focused events this week, and he often shows up on picket lines and worker rallies.
"I don't think there's hesitancy over John Edwards' candidacy. I think that union leaders want to make sure that whatever direction they recommend on endorsement that they really know where their own workers stand," Sweeney said.
"There's no question that John Edwards has been raising the issues that are very close to the hearts of workers and has been I think motivating some of the debate that's going on, on those issues, but we have other candidates that are articulating well on those issues that are important to workers. But it's safe to say that any one of the seven candidates would be a hell of a lot better than the present occupant of the White House." E-mail to a friend
CNN's Candy Crowley contributed to this report.