In what one lawmaker described as the final act of a tedious drama, the House defied President Reagan's veto threats and approved separate plant closing and trade reform bills by overwhelming margins.

Representatives first passed the plant closing measure 286-136, more than the two-thirds majority needed to override an anticipated veto, and then rallied 376-45 Wednesday for the sweeping trade reform legislation.Aboard Air Force One Thursday, White House spokesman Marlin Fitzwater told reporters the president's "senior advisers recommend a veto and it (the plant closing bill) was a major reason why he vetoed the (original) trade bill."

Fitzwater said the administration is "for the creation of jobs (but) we believe this bill stymies job creation." While the concept is supported at the White House, he said, it ought to be in labor contracts, not on the law books. Fitzwater denied that a veto would be politically damaging to Vice President George Bush's bid for the presidency.

Ways and Means Committee Chairman Dan Rostenkowski, D-Ill., described the reform bill as "a powerful signal that this country will be a forceful but fair leader in the search for open and expanded world trade."

Even many Republicans who criticized the bill as imperfect conceded it represents the last chance to implement needed trade reforms before a new administration takes office.

Rostenkowski, who steered the legislation through the House during the past few years only to have it vetoed, added: "After a long and tedious process, I sense that we are at last approaching the final act in this drama."

Congress had approved the trade and plant closing measures this spring as a single package that was sent to the White House. But Reagan vetoed the effort May 24, objecting to the provision requiring companies to give advance notice of factory shutdowns and major layoffs.

The House voted 308-113 the same day to override the veto, but the Senate failed to muster the two-thirds majority to enact the legislation over the president's objections. Democratic congressional leaders then decided to separate the bills and try again.

Senate Democratic leader Robert Byrd of West Virginia has warned he will not bring up the trade bill for Senate consideration until Reagan signs the plant closing bill. If the president vetoes it, Byrd said the plant closing language - now a keystone of the Democratic presidential campaign - will be re-attached to the trade bill.

The plant closing measure requires companies with more than 100 employees to give 60 days' notice before closing a factory or laying off one-third or more of the work force. Businesses would be required to give back pay to employees and $500 a day to affected communities for each day violating the time frame.

"Sixty days notice can make a difference in the life of a worker, a family, a community," said Rep. William Clay, D-Mo., taking a party line Wednesday. "It is a basic decency afforded by most other countries and . . . it is legislation that is a must for this Congress."

Democrats, including presidential candidate Michael Dukakis, are playing the question as one of simple fairness, seizing it as a major campaign issue.