WASHINGTON -- With a strike deadline near, negotiators for UPS and the Teamsters union returned to the bargaining table with a federal mediator Thursday night to try to prevent a midnight shutdown of the delivery service.

UPS, based in Atlanta, has 302,000 U.S. employees, about two-thirds of them Teamsters. Only the U.S. Postal Service employs more workers under a contract.

About 60 union members who work at the UPS facility near the Cape Girardeau Regional Airport were poised to strike Thursday night. They belong to the St. Louis-based Local 688. Most of them are local delivery drivers, said Mike Goebel, a business representative for Local 688 who handles UPS workers.

After a brief session with the full bargaining committees from both sides, John Calhoun Wells, director of the Federal Mediation and Conciliation Service, retired to a hotel room with the lead negotiators from each side to try to kick-start the stalled talks.

"They have returned to the table," said Steve Trossman, a Teamsters spokesman. "We are committed to doing what we can to reach a contract that meets the needs of Teamsters members. But if we don't reach agreement, we're prepared to strike."

After what union officials said were productive meetings in recent days, the talks stalled when company negotiators abruptly presented what UPS said was its final offer late Wednesday night.

"We look at this as another bargaining tactic," said Teamsters spokesman Rand Wilson, who welcomed the mediator's participation. "We will not agree to this contract."

As the clock ticked toward a midnight Thursday deadline, union representatives waited for UPS negotiators at a Washington hotel.

"We have made a last, best and final offer to the union," UPS spokesman Ken Sternad said earlier. "We put it on the table late last night when it became apparent that there was not going to be an agreement."

Nevertheless, Sternad said the company remained open to discussing its proposal with the union.

Goebel said the key issue is job security. He said 60 percent of UPS employees nationally are part-timers, and that number is rising.

Over the life of the four-year agreement that expired at midnight, 83 percent of those hired were part-timers, Goebel said. "Rather than leveling off, the UPS offer would increase the number of part-timers," Goebel said.

He said UPS's final offer would limit the number of part-timers who could become full-time to 10,000 over the life of the contract. He said many of the new-hires would be on-call employees who wouldn't know from day to day how much they would work.

Except for part-time sorters, most employees in Cape Girardeau are full-time drivers, he said.

Goebel said that in cities like St. Louis, UPS has to hire four people for each position a year. "It used to be you worked hard and they rewarded you for it," he said. "Now there's no future, so people don't stay."

UPS claims it had created 1,000 new full-time jobs and given part-timers a leg up in applying for full-time posts when they become open.

Another sticking point was the company's insistence on ending its participation in the union's multi-employer pension and health funds to establish a separate UPS benefits pool.

The company said that in addition to a modest wage increase, its offer included a $3,060 bonus for full-time employees and $1,530 for part-timers. If profit goals are met, the bonuses would be repeated in two years.

Full-time UPS drivers earn $19.95 an hour on average. Part-timers start at $8 an hour, the same starting wage as 1982. The company offer calls for keeping it at $8.

Goebel said wages are not a major issue.

A UPS message to customers Thursday said, "We are advising our customers that after the contract expires, delivery delays may be possible." The company had sought a contract extension, but the Teamsters declined.

UPS' 2,000 pilots, represented by the International Pilots Association, have pledged to join the Teamsters in a walkout.

Competitors such as Federal Express Corp. and the U.S. Postal Service were making contingency plans, but analysts say it would be difficult to make up UPS' volume of 12 million parcels and documents a day.

The Teamsters waged an aggressive campaign to build support for a strike if needed, but on Thursday the company distributed leaflets at work sites, alleging that union leaders were blocking a good deal.

The Teamsters' Wilson said the company had already "moved closer to meeting our members concerns" in informal talks between lead negotiators,

"We expect them to come back to the table and get real," he said.

White House spokesman Mike McCurry said top Clinton aide Bruce Lindsey had met with negotiators. "It's been mostly an informational session ... so that the White House will be abreast of collective bargaining discussions they've been having," McCurry said.

(Some information for this story was provided by The Associated Press.