WASHINGTON (AP) _ Democratic congressional leaders urged union representatives Wednesday to accept plans to drop a key plant-closings provision from major trade legislation but made no headway, sources said.

''We're still searching,'' House Speaker Jim Wright, D-Texas, told reporters after the session. He refused to comment on details of what happened in the closed-door meeting.

Representatives of the United Auto Workers union and other labor groups were warned at a three-hour meeting that the plant-closings measure could scuttle the whole bill, according to sources who asked not to be identified by name.

Reagan administration officials have said that the president most likely would veto the trade bill if it arrived at the Oval Office containing the heavily debated plant-closing provision.

The provision in question would require companies with at least 100 employees to provide 60-day notice of plant closings and layoffs involving at least one-third of their work force. Layoffs involving 500 workers or more would have to be preceded by notice even if less than one-third of the work force were not involved.

Democratic leaders reviewed with the labor representatives provisions of the bill they said would help to preserve blue-collar jobs in the face of intensified foreign competition and providing increased aid to those left unemployed as a result of stepped up imports, the sources said.

Rebuffing that argument, the labor representatives called on congressional leaders to hold firm against pressure to drop the plant-closings provision and pass the bill with the measure intact in defiance of the veto threats, the sources said.

The labor representatives left the session in the office of Senate Majority Leader Robert C. Byrd, D-W.Va., without commenting to reporters as did a number of congressional participants.

Other congressional participants were House Ways and Means Committee Chairman Dan Rostenkowski, D-Ill., Senate Finance Committee Chairman Lloyd Bentsen, D-Texas, and Sen. Edward M. Kennedy, D-Mass.

Industry groups have repeatedly attacked the measure as burdensome and perhaps merely the first step toward additional measures that would take control of businesses away from their managements.

Central provisions of the trade bill would transfer to the U.S. trade representative authority to retaliate against international violations by the nation's trading partners and other ''unjustifiable'' practices. They also would streamline machinery for imposing import curbs to aid threatened U.S. industries.

But the 1,000-page bill also contains a wide array of other provisions ranging from repeal of the windfall profits tax to tighter monitoring of lamb imports.

Lawmakers have been searching for a formula that would guarantee that President Reagan would sign the bill rather than wield his veto pencil. The issue has boiled down to the plant-closings provision.

Wright hinted earlier Wednesday that the leaders were leaning toward dropping the provision when he told reporters that it would still represent ''a good bill'' if shorn of the plant-closings feature.

''It would be a better bill with that provision'' included, he added. But he gave reporters a list of items that should be in the legislation and omitted plant-closings.

Dropping the plant-closings provision, however, could prove to be a difficult job, even for Democratic leaders.

Opposition from labor unions could force the issue into the spotlight and complicate relations between the leadership and such plant-closings sponsors as Sen. Howard M. Metzenbaum, D-Ohio, and Rep. Augustus F. Hawkins, D-Calif.

Kennedy, the only plant-closings sponsor in the afternoon session, was noncommittal during one brief break on whether he was making any progress in saving the measure. He said merely: ''We'll see.''

The plant-closings issue triggered a testy exchange Wednesday between Labor Secretary Ann McLaughlin and Sen. Lawton Chiles, D-Fla.

Mrs. McLaughlin, in defending her department's proposed $6 billion budget for next year, drew a sharp rebuke from Chiles, chairman of the Senate Appropriations Committee's panel on labor, health and human services and education.

Chiles cited a letter from Mrs. McLaughlin to Byrd in which she said she would urge Reagan to veto the trade bill if it included the provision.

''Lots of workers are told nothing,'' Chiles said. ''Isn't there some equity in putting an employee on a little notice that something's happening, that their plant's closing and they will be without a job?''

Mrs. McLaughlin said the notice provision would ''decrease flexibility at the plant level'' and, in some cases, might actually trigger a plant closing that otherwise would not occur.

She acknowledged ''there are some'' instances where workers receive no warning they are losing their job but added, ''It's not the majority of cases.''

The Bureau of Labor Statistics, in a study last year, estimated that two out of every three workers are not told they are losing their job until the day they are taken off the payroll.

The National Academy of Sciences said in a report last June that the voluntary notices advocated by the administration and business do not now provide workers with the ''minimum two or three months necessary to adjust successfully to job loss.''