The 13.1 million-member AFL-CIO endorsed Vice President Gore's bid for the presidency today, giving his struggling campaign a huge infusion of manpower, organization and money -- and, most important, direct access to union households making up a quarter of the Democratic primary electorate in key battleground states.

Two major unions -- the Teamsters and the United Auto Workers -- abstained in protest of administration free-trade policies, though in recent weeks the White House has moved to emphasize its commitment to addressing labor issues in trade pacts. Officials expect both unions to eventually endorse Gore, but a key political question is whether the lingering conflicts over trade issues will mean that the labor federation's backing for Gore will not be full-bore and enthusiastic.

Securing the AFL-CIO's backing was considered the "first primary" by both Gore and his main rival, former senator Bill Bradley of New Jersey. Not only is the endorsement a political boost for Gore, who has been slipping in the polls, but it means he will almost instantly obtain an array of phone banks, precinct workers, communications networks and local advocates in key primary states.

In Iowa, which holds the first caucuses in the nation 15 weeks from now, for instance, labor will put 40 campaign workers in each congressional district. Statewide, there are 160,000 union members.

Polling now suggests that Gore is the favorite in Iowa and Bradley in New Hampshire, where labor is weaker. If Gore and Bradley split those states in early February, then it will put labor to the test in New York and California, where the nation's largest blocs of union workers are located in the New York City and Los Angeles areas. New York and California will hold their primaries shortly after New Hampshire, on March 7, along with at least nine other states.

The only other time labor has endorsed this early in the process was 1983, when the beneficiary was former vice president Walter F. Mondale. In that contest, labor played a crucial role in keeping the Mondale campaign afloat during a brutal challenge by Gary Hart, who unexpectedly beat Mondale in the New Hampshire primary. Mondale went on to win the Democratic nomination.

Under the leadership of John Sweeney, the AFL-CIO has reemerged as a major political force. Since the Democrats lost control of Congress in 1994, organized labor has played a key role reviving party fortunes, providing the margin of victory in a host of statewide and congressional races in the elections of 1996 and 1998. Sweeney and other union leaders are determined to prevent a Republican lock on Congress and the White House after the 2000 elections, preparing to spend $40 million on union mobilization in key centers of labor strength.

Gore and his campaign aides were ecstatic over the labor federation's decision.

"I stand with you, I will fight for you," Gore told the cheering convention delegates. The vice president moved immediately to assuage conflicts with industrial unionists over trade, reiterating administration policy that future trade agreements will have key labor and environmental provisions in the documents, not "side" agreements as in the case of the North American Free Trade Agreement. "I will insist on workers' rights, human rights and environmental standards as part of the agreements," Gore promised the convention.

Tonight in Washington, President Clinton also said he will put labor and environmental issues higher on the agenda in future trade talks. (Details, Page E1.)

"This was a fundamental test of strength at a critical point in the nomination process," said Gore strategist Tad Devine. "We have a contest centered on fighting for working families; to have the support of the organizations that represent those families is one of the most critical endorsements to have," he said. "The party that wins households [with incomes] between $30,000 and $50,000 will be the party that wins the election for president."

Bradley, who had fought to delay the endorsement, issued a conciliatory statement, saying he respected the federation's decision and that his "commitment to working men and women and the role that labor can play in their lives is unwavering." The AFL-CIO, in announcing the endorsement, was careful to tip its hat to Bradley and his "good record" on labor issues.

Teamsters leaders voiced the only public opposition to the early endorsement. James P. Hoffa, the president, told the convention: "I understand the need to lead, but there is also a need to listen. Now is the time to listen." Jerry Vincent, a Kentucky Teamsters delegate, told representatives of the 68 member unions in the federation: "In the South, we always say, `Don't wrap that pig, weigh it.' And we'd like to have it weighed before it's wrapped."

While the Teamsters are expected to eventually endorse Gore, Hoffa aides said today he would also seek to meet with the GOP front-runner, Texas Gov. George W. Bush.

Republican Party officials immediately moved to try to drive a wedge between the unions that led the charge in support of Gore -- largely, public employee organizations -- and the industrial unions that were most opposed to the early endorsement.

"Today's split decision does expose the fault line between labor leaders representing America's traditional industrial and service employee unions . . . and the union bosses whose base consists of government bureaucrats. In the labor movement today, workers who pay taxes are being held captive by those who spend them," said Republican National Committee Chairman Jim Nicholson.

Swoboda reported from Los Angeles, Edsall from Washington.