By STEVEN GREENHOUSE

LOS ANGELES -- The nation has lost 500,000 manufacturing jobs over the last 18 months, despite the economic boom, and organized labor's eagerness to see that trend reversed is shaping up as an economic test for President Clinton and a huge political test for Vice President Al Gore.

Union leaders say that even though Gore won the endorsement of the AFL-CIO at  its convention here, the administration will need to take strong steps toward stopping the loss of manufacturing jobs if the vice president hopes to excite rank-and-file workers to vote for him.

The job losses have become perhaps the most important issue to labor unions in  six large industrial states that are crucial swing states in presidential elections: Illinois, Michigan, Missouri, Ohio, Pennsylvania and Wisconsin.

Many officials from the powerful industrial unions in those states -- the auto workers, the steelworkers and the machinists -- say that if Gore hopes to motivate union activists to campaign for him and union members to vote for him in primaries and in the general election, he must address labor's concerns on manufacturing and the related subject of trade.

"It's one thing for the vice president to get the AFL-CIO's endorsement, and it's quite another to mobilize our members," said George Becker, president of the United Steelworkers of America. Becker met with Gore last week to press him on trade and manufacturing. "I have one overall goal in life," Becker said, "and that is to reverse the trend of de-industrializing America and to stop these insane trade laws that force our manufacturers to compete against impossible odds."

The International Brotherhood of Teamsters, which is a force in the Middle West as well as in two important early primary states, New York and California, is    also warning the vice president that the administration needs to do more to stop industrial decline. The teamsters and auto workers were the only unions to oppose the AFL-CIO endorsement of Gore, and both unions are demanding firmer commitments from the administration and the vice president on trade if they are  to back him.

Mindful of these pressures, Clinton and Gore have called for some labor-friendly provisions in trade treaties and have created a task force to study the plight of manufacturing.  While labor leaders are encouraged by these steps, they say
the important test will be what specific actions the administration takes -- and whether Gore will disagree with the administration if he feels it does not go far enough.

Complicating matters for the vice president, many middle-of-the-road Democrats and business executives are cautioning the administration and Gore not to become tools of labor and not to comply with demands that they deem protectionist.

Labor unions, which are considered vital to turning out the Democratic base in general elections, have a long wish list for the administration, and they hope the vice president will press labor's viewpoint.

Union leaders say that whatever reservations they have about the administration and Gore on trade, the views of former Sen. Bill Bradley, the vice president's only rival for the Democratic presidential nomination, are no better.

Unions want Clinton and Gore to make sure that any future trade agreements include penalties against trading partners that allow violations of minimal international labor standards, like child labor rules. Unions oppose admitting China to the World Trade Organization, saying a country that uses prison labor and bars independent trade unions should not be given more access to the U.S. market.

While avoiding their blatantly protectionist rhetoric of a few years ago, labor leaders have attacked the record trade deficit, which may top $300 billion this   year. They also dislike the strong dollar, which has cost U.S. manufacturing       jobs by making American goods more expensive overseas and by making foreign goods cheaper.

Christopher Lehane, a spokesman for the Vice President's campaign, said, "Al Gore agrees with the many representatives of labor, including those from the industrial unions, that there is nothing more important that we can do to make sure we can expand our prosperity to all portions of the country, and that includes, most prominently, our industrial sector -- our steelworkers, our auto workers, our truck drivers, our shipbuilders."

The 1.4 million-member teamsters union has its own specific demand, that the administration stop a provision in the North American Free Trade Agreement from taking effect on Jan. 1. That provision would lift restrictions that bar Mexican truckers from driving in the United States.

Pushing for continued restrictions on cross-border trucking, teamster officials argue that most Mexican trucks are unsafe. They fear that a huge influx of low-wage Mexican drivers would wipe out the jobs of thousands of American truckers.

In a warning to Gore, Michael Mathis, the teamsters' political director, said, "I think we'd be very much less excited about Gore if the border is open Jan. 1.   We'll be very, very disappointed."

Many union leaders have told the vice president to heed a lesson from 1994, when  the Democrats lost control of the House. That year, union members were so angry with Clinton and the Democrats over passage of the North American Free Trade Agreement that an unusual number of union members decided not to vote, giving  the Republicans an edge in many districts.

An early test of whether the administration and Gore will pass labor's scrutiny on trade will come in Seattle on Nov. 30, when the World Trade Organization's ministerial meeting begins. The meeting is expected to set ground rules for a new multiyear round of talks aimed at liberalizing world trade.

The unions want future trade pacts to include basic labor protections and to give countries the right to impose penalties against nations that violate minimum    labor standards, like a ban on child labor. But many companies and third-world countries say labor-related provisions have no place in trade agreements, and they are pressuring Clinton and his vice president to ignore labor's pleas.

"They got to be very careful about how they handle Seattle, notwithstanding labor's  endorsement of Gore," said Jay Mazur, chairman of the AFL-CIO's international affairs committee and president of the Union of Needletrades, Industrial and Textile Employees. "Remember, our people are upset about losing 500,000 manufacturing jobs."