WASHINGTON (Reuters) - Democratic presidential front-runner John Kerry received the endorsement of the 13 million-member AFL-CIO labor coalition on Thursday, a move that could help him rebut rival John Edwards' challenge on jobs and trade.

"Today we know the time has come to unite behind one man, one leader, one candidate," said John Sweeney, president of the AFL-CIO, an umbrella organization of 64 unions. "Throughout his distinguished political career John Kerry has been a friend of working families."

Outside AFL-CIO headquarters two blocks from the White House and beneath a huge sign reading "America Needs Good Jobs," Kerry vowed to fight for working families and ridiculed President Bush's "fuzzy math" on jobs.

He said every year Bush had promised to create jobs and every year he had lost them -- the economy has shed almost 3 million in the private sector since the president took office -- and "now he says he's not in charge of numbers."

White House advisers have distanced themselves from their own forecast that the country would add 2.6 million jobs by the end of the year.

"Well, ladies and gentlemen, it just doesn't take a lot of fuzzy math to count to zero," Kerry said. "We're not asking George Bush to count the jobs, we're asking George Bush to create the jobs and fight for working people."

The loss of U.S. jobs to foreign competition has become a focal point in the Democratic race to challenge Bush and looms large in states like Ohio, New York and Georgia, which are among 10 holding contests on March 2.

Kerry has won 15 of the 17 primaries and caucuses so far and is the clear leader in money and momentum. The four-term Massachusetts senator has a decent chance of wrapping up the nomination in the next two weeks despite Edwards' good showing in Wisconsin on Tuesday.

Job losses in manufacturing and sending work overseas shot to the front-burner in recent weeks when the Democratic presidential competition moved to states like Michigan, Wisconsin and Missouri.

NAFTA BECOMES AN ISSUE

Edwards, who finished a surprisingly close second in Wisconsin and turned the race into a two-man battle, has challenged Kerry on his 1993 vote for the North American Free Trade Agreement, implying that he cost Americans jobs.

In a coup for Kerry, the AFL-CIO buried any differences over free trade and backed him in the interest of beating Bush. The endorsement by the politically active AFL-CIO gives him valuable organizational muscle and a new source of grass roots support.

"This is man who will not sign his name to a single trade agreement that does not include labor protections and environment protections," Sweeney declared.

The AFL-CIO held off endorsing anyone in the early primaries despite a major effort by Rep. Richard Gephardt of Missouri to get the union group to back him.

Kerry has countered Edwards by pointing out that he and the first-term North Carolina senator, who was not in Congress when NAFTA passed, have indistinguishable positions on trade and that they both voted for the China trade pact.

Edwards hopes to use his opposition to NAFTA and his southern working class roots to highlight differences with Kerry, a New Englander from a privileged background, and open up the economic debate, especially in Ohio, Georgia and upstate New York which are suffering severe unemployment.

Edwards kept to his economic theme in a speech at Columbia University in New York on Thursday, deploring the loss of jobs overseas and claiming Bush only cared about Wall Street and not those struggling to find work.