General Motors and the United Automobile Workers were working Saturday to hammer out a new labor agreement in the final hours before their current contract expires.

G.M. is seeking to lower health-care costs and other expenses, while the union hopes to get the automaker to produce more vehicles at United States plants and to reopen a factory in Lordstown, Ohio, that was idled in March.

The union is focusing its negotiating efforts on G.M., while putting talks with Ford Motor and Fiat Chrysler on hold. The four-year contracts with each manufacturer expire at 11:59 p.m. Saturday. On Friday, the U.A.W. agreed with Ford and Fiat Chrysler to extend their contracts indefinitely as the union bargains with G.M.

On Saturday, the U.A.W. said in a letter to members that it would not extend the G.M. contract and would continue working without a new bargaining agreement as talks continued. If the two sides fail to reach a tentative agreement, the union could strike at G.M. plants. “We are united in our efforts to get an agreement our members and their families deserve,” a U.A.W. vice president, Terry Dittes, said in the letter.