The United Auto Workers put Chrysler on notice that a strike is possible if contract talks stall, a person briefed on the talks said, but a labor expert said the union's action could be a bargaining tactic.
The union on Sunday gave Chrysler a 72-hour notice of a potential strike, the person said, but it was unclear exactly whether the notice would end on Tuesday or Wednesday.
Bargainers working in committees made progress during the weekend but still have much work to do on difficult issues, said the person, who asked not to be identified because the talks are private.
Negotiators bargained Saturday and into Sunday evening, then recessed for the night with plans to resume talks Monday.
"We remain optimistic," Chrysler spokeswoman Michele Tinson said Sunday afternoon.
UAW spokesman Roger Kerson declined to comment on the talks.
A strike notice could be a tactic by the union to put pressure on the company as the talks intensify, said Harley Shaiken, a professor at the University of California at Berkeley who specializes in labor issues.
"The union wants the deadline to encourage a settlement sooner rather than later," Shaiken said.
The UAW went on strike for nearly two days last month before coming to a tentative agreement with General Motors . The union normally settles with one U.S. automaker and then uses that deal as a pattern for an agreement the other two. But this year, both Chrysler and Ford Motor have said they have different needs than GM and may need different contract terms.
Shaiken said the notice doesn't necessarily mean there will be a strike because the UAW could extend its contract hour-by-hour when the deadline passes. A second strike in one set of negotiations would be rare, he said.
"I think the union may feel things are going well, but they want the discipline of a deadline," Shaiken said.
A short strike might not hurt Chrysler much. It has five U.S. plants scheduled to be shut down for a week or two starting Monday due to lower market demand for their products.
GM workers are now voting on the tentative agreement reached with the company, with totals expected to be done on Wednesday.
The union has not formally picked the second company it will negotiate with, but talks with Chrysler have intensified in recent days.
The UAW's contracts with Chrysler, Ford and GM were originally set to expire Sept. 14. The UAW chose GM as the lead company and strike target and reached a tentative agreement Sept. 26.
The UAW represents about 49,000 hourly workers at Chrysler, making it the smallest of the domestic automakers. The company also has about 78,000 retirees and surviving spouses represented by the UAW.
Chrysler has become a private company, which could be a factor in the talks. Private equity firm Cerberus Capital Management bought a majority share of Chrysler in August from DaimlerChrysler. As a private company, Chrysler no longer has shares and isn't required to file earnings reports.
Chrysler pays its workers an average of $75.86 per hour in wages, pension and health care costs, the highest among the Detroit automakers