Holding enough votes this time to override a veto, Democrats are forcing a second election-year showdown with President Reagan over giving workers advance notice of plant closings and large layoffs.

On a 72-23 vote Wednesday, the Senate passed a bill requiring companies to give 60 days' notice before shutting down any operation or instituting a layoff affecting more than 50 people or a third of the work force.President Reagan cited the mandated notices as his primary reason for vetoing a massive overhaul of the nation's trade laws two months ago, saying advance notices would reduce U.S. competitiveness and stifle new job creation.

But with polls showing more than 80 percent voter support for the measure and Democrats using the issue to attack the economic policies of Reagan and Vice President George Bush, 19 Republicans - including seven up for re-election - broke with the White House on Wednesday's vote.

Wednesday's vote was the biggest legislative victory for organized labor since it helped return the Senate to Democratic control in the 1986 election after Republicans held the majority the first six years of Reagan's presidency.

Just nine Senate Republicans supported the mandatory notices a year ago, when they were made a part of the trade bill. And only 10 broke ranks last month when the Senate, on a 61-37 vote, fell five votes short of the necessary two-thirds for overriding Reagan's veto.

A White House official said after the vote that Reagan has not stated definitely that he will veto it. The official, speaking on the condition of anonymity, said the president's advisers were divided on the issue even though the administration put out a statement prior to the vote saying they would urge him to veto it.

"I don't know what the president's going to do when he gets this bill," said Senate Minority Leader Bob Dole, R-Kan., who complained earlier about labor leaders "wielding a lot of clout in this body."

Both supporters and opponents of the legislation raised the possibility that Reagan might allow the bill to become law after the expected House approval without signing it. Under the Constitution, a president has only 10 working days to veto a bill after passage by both houses.