Washington ? Divided for much of the political season, organized labor got behind the candidacy of Democratic front-runner John Kerry on Thursday, with the president of the AFL-CIO proclaiming that it was time “to unite behind one man, one leader, one candidate.”

Bestowing the endorsement on Kerry at an outdoor rally barely a block from the White House, AFL-CIO President John Sweeney said his organization, comprised of 64 unions representing more than 13 million U.S. workers, would mobilize in unprecedented numbers for the man they figure will be the Democratic nominee.

“John Kerry will lead us in our fight to make creating good jobs America’s number one priority,” Sweeney said, his focus on ousting President Bush in November.

All the talk of unity was in sharp contrast to the fate of organized labor in recent months. Unable to settle on one candidate, the AFL-CIO withheld its endorsement late last year, a reflection of the fissures among the unions.

More than 20 of the international unions backed Rep. Dick Gephardt of Missouri, the presidential candidate who had carried labor’s water in Congress for nearly three decades. But the largest unions — the American Federation of State, County and Municipal Employees and the Service Employees International Union — stunned the Democratic race by endorsing Howard Dean.

When the first votes were counted in Iowa, labor proved to be the big loser — along with Gephardt, who finished a poor fourth and exited the race, and Dean, who was a disappointing third. The political influence of labor and its voter-turnout operations appeared to be waning.

The labor vote has been critical to the Democrats, with union members voting for Al Gore over George Bush by about a 2-to-1 margin in 2000, according to exit polls.

“The lesson clearly is, when we’re united we’re a lot stronger than when our support is divided among four candidates,” said Andy Stern, president of SEIU, the federation’s largest union with 1.6 million members.

In this year’s Democratic primaries, those from labor households have made up anywhere from a fourth of the vote to a third in states such as Delaware, Iowa, Missouri and Wisconsin, according to exit polls conducted for The Associated Press and the television networks by Edison Media Research and Mitofsky International. Those voters tended to support Kerry, by narrow margins in Iowa and Wisconsin, and by substantial numbers — 20 to 40 percentage points — in Missouri and Delaware.

“Today we stand united in a common cause and that common cause is not just to defeat George Bush, but it is to put our country back on track, on the road of prosperity, the road of fairness, the road of jobs,” Kerry said.