Senate Democrats on Friday overwhelmingly defeated a pair of Republican amendments to a bill requiring companies to give advance notice of layoffs and plant closings. A showdown on the issue was delayed until Monday.

Defeated on respective 60-27 and 59-31 votes were amendments to make picket line violence during a labor dispute a federal crime and to exempt from the notice requirements employers who give laid off workers 60 days severance pay.After stalling action on the legislation for three days, Republican leaders agreed Friday to allow a vote late Monday on their main attempt to derail the notice requirements, which were separated from a trade bill that President Reagan vetoed last month because of them.

Senate Minority Leader Bob Dole, R-Kan., said the chief GOP effort will be through an amendment to be offered by Sen. Nancy Kassebaum, his fellow Kansas Republican, to either eliminate entirely the 60-day notice requirement for layoffs or shorten it.

Mrs. Kassebaum, who was away from Washington on Friday, said earlier she supports 60 days notice in cases of plant closings but not when an employer is just laying off 50 or more workers, as the Democratic-sponsored bill requires.

But in a showdown with the Reagan administration, Democratic leaders repeated their threat to kill to any chance of winning enactment of either the advance notices or the trade bill if Mrs. Kassebaum's amendment is approved.

"This is a killer amendment," said Majority Leader Robert C. Byrd, D-W.Va. "Not only does it kill this bill but it will kill the trade bill. I've maintained all along that the president doesn't want a trade bill."

Democrats still hoped to send the plant-closing measure to the White House by July 4 for an expected veto by Reagan.

Byrd said Friday he anticipates final passage of the plant-closing bill by the Senate on Tuesday. House Speaker Jim Wright, D-Texas, has vowed to seek its passage in the House immediately upon the Senate's approval.

Polls showing that more than 80 percent of Americans advocate advance notices of plant closings and layoffs have emboldened labor leaders to threaten withdrawal of their support for the trade bill if the measure is not enacted.

Sen. Strom Thurmond, R-S.C., attempted to attach the picket line violence measure to the bill. The amendment would have overturned a Supreme Court decision stating that such violence is not a violation of federal anti-extortion laws.

"I'm sick of seeing a union go into a man's house and break four of his ribs because he doesn't strike with them," Thurmond said. "If the Senate wants to end violence on the picket line, now is the time to do it."

The opposition to Thurmond's amendment was led by fellow Republican Sen. Arlen Specter of Pennsylvania, who called it a states' rights issue.

"There are adequate provisions under the law in all 50 states to see that this kind of violence is prosecuted," Specter said. "To bring in federal marshals and U.S. attorneys for what may be only a fistfight would put a chilling effect on First Amendment rights to free speech."