It is a tough call for the unions, analysts say, because the eight Democrats are all friends of labor.
Unions still are feeling burned after they went out on a limb early in the 2004 contest, only to watch as their anointed candidates crashed and burned.
The 2008 candidates "might like labor to come out and endorse in the primary, but the labor movement is only going to do that if they see some great advantage for them," said Paul F. Clark, head of the department of labor studies and employment relations at Penn State University. "Not all unions think that that is something that will benefit them."
Leading the pack of fence-sitters are politically powerful unions such as the International Brotherhood of Electrical Workers, which gave Democrats $3.1 million in the 2006 elections, the most of any union.
The Service Employees International Union recently disappointed the leading contenders, who had sought its money and foot soldiers, by declining to make a national endorsement.
The International Brotherhood of Teamsters, one of the largest unions in the world and a powerful player in the Democratic Party, is uncommitted in the race, too.
The AFL-CIO, the nation's largest labor federation, has not found any one candidate to be so much better than the rest that it could justify an endorsement before the first primary.
Instead of trying to be kingmakers, these unions plan to wait for a nominee to emerge before they spend some of their substantial resources in hopes of putting a Democrat back in the White House in January 2009.
"We feel that we want to save our energy for the general election," said Jim Spellane, spokesman for the electrical workers union. Added service employees union spokeswoman Stephanie Mueller: "Once a Democratic nominee is chosen, we will put forward the largest grass-roots program ever for a presidential campaign."
Former Sen. John Edwards of North Carolina, Sen. Hillary Rodham Clinton of New York, Sen. Barack Obama of Illinois and other candidates have telephoned both national and local union leaders to make the case for why each of them deserves an endorsement.
Clinton has the most national union endorsements so far with six, while Edwards has four. Sen. Christopher Dodd, D-Conn., was endorsed by the International Brotherhood of Firefighters. Obama has an endorsement from a New York City correction officers union.
Edwards, meanwhile, has been endorsed by 11 SEIU state chapters and Obama by two.
Many union plums await picking. For example, the American Federation of State, County and Municipal Employees plans to endorse within a month.
The National Education Association and the Communications Workers of America will decide in December whether to endorse or to endorse more than one candidate. The United Auto Workers has yet to make its plans known.
Sitting on the sidelines is easier this election because all the Democratic candidates support organized labor. The failed presidential runs of Howard Dean and Dick Gephardt also remain fresh in the minds of union leaders.
Unions were burned during the 2004 Democratic primaries after they supported Dean and Gephardt, only to see their campaigns wither. The nomination went to Massachusetts Sen. John Kerry, who had scant union support in the beginning. He lost to President Bush.
Once bitten, twice shy, says Robert Bruno, a professor at the Institute of Labor and Industrial Relations at the University of Illinois at Chicago.
"Nobody wants to be turning out all of their national resources on behalf of a candidate who may not be the party's ultimate choice," he said.
The leading candidates in 2008 - Clinton, Obama and Edwards - have enthusiastic support inside just about every union. The other candidates also push union issues at every turn, making it hard to back one candidate without causing a rift in the union.
"If you were to endorse one on behalf of the entire union, then those segments of the union who are enthusiastic about another candidate would be put in an awkward position," Clark said.
For the service employees union, the first to go public with its indecision, that reflects "that there are a number of candidates on the Democratic side who will stand up for workers," Mueller said.
The same thing happened at the electrical workers union, Spellane said. "We find that our members are all over the place," he said.
Clark said it will be hard to get undecided unions off the fence before voting begins.
Edwards, for example, has pinned much of his presidential hopes on getting support from organized labor, Clark said. Edwards has spent considerable time the past couple of years walking picket lines, speaking out for workers' rights and seeking labor support.
"If Edwards can show some traction, then he's going to get more attention from the labor movement," Clark said. "The problem is, he may well need the labor movement to generate that traction."