Union President Leo Gerard extended the endorsement to the Missouri congressman, saying he was "not just labor's guy," but the "heart and soul and conscience" of America's workers.
The union endorsement is his 11th, following a coveted seal of approval from the Teamsters (search) in a leadership vote last week.
Union officials are meeting this week in part to determine whether Gephardt, who has staked his campaign on the support of organized labor, can snag enough endorsements from individual unions to win the AFL-CIO (search) prize.
The AFL-CIO has endorsed only twice before: Al Gore in the last election and Walter Mondale in 1984. It's a difficult benchmark, requiring support from two thirds of the AFL-CIO's 13 million rank-and-file members.
Labor leaders say Gephardt is the only current candidate with a chance to join that list, even as they caution that there is a real possibility the labor group will make no endorsement.
The steelworkers' endorsement comes as unions' share of the work force has declined every year for the past 20 years, yet the power of organized labor at the polling place continues to grow.
Mindful of labor's election muscle, the nine Democratic presidential hopefuls are courting union leaders gathered for the AFL-CIO's executive council meeting, and also will participate in a Tuesday night forum.
Each is hoping to win labor's crown -- a political mobilization machine that turns out voters.
For while the proportion of union members in the work force has fallen from 20 percent in 1983 to 13.2 percent now, labor's relative strength at the ballot box continues to grow in the face of overall declines in voter turnout.
The share of union household voters has grown from 19 percent in 1992 to 23 percent in 1996 to 26 percent in 2000.
The AFL-CIO may call for another meeting later this fall, giving them more time to gauge support.
"I think it's going be hard because there are a bunch of big unions, particularly some of the service and public sector unions, that aren't ready to endorse him," said Steve Rosenthal, the former AFL-CIO political director credited with building labor's successful political program.
Questions plague the Missouri congressman's campaign, particularly about whether he can win. Gephardt, a longtime ally of labor, also sought the nomination in 1988.
For the United Food and Commercial Workers (search), which had its convention last week, members are widely divided about who to support.
"What's become clear from our convention is there is no consensus in our union for any one candidate," said union spokesman Greg Denier. Former Vermont Gov. Howard Dean (search) and Gephardt "both got tremendously good responses," he said, "but there was not some clear favorite, that this is the person."
Another critical issue facing organized labor this week is money. With most unions struggling in the poor economy and membership down in some of them, labor officials are trying to figure out how to finance the largest political effort ever.
"After seeing George Bush raise 200 million dollars, I think we know that to be able to talk to our members and in some cases nonunion voters, we're going to need a lot of resources to be heard," said Andrew Stern, president of the Service Employees International Union (search), the largest in the AFL-CIO.
The executive council will consider transferring about $5 million from the AFL-CIO's general fund or organizing budget, and levying another surcharge on its affiliates of 4 cents per member per month to boost its political funds.
The same surcharge was approved for the 2002 election cycle - after much grumbling from some unions - and was to pay for the political program through 2005.
The AFL-CIO spent $42 million in the last presidential cycle and Stern said the council is considering an increase of about 25 percent.