By Saturday, the union and the company appeared to have reached agreements on most major issues, including wage increases and a path for temporary workers to become permanent employees, the people close to the talks said. Two unresolved matters, they said, included the time of service required for less senior workers to reach the top union wage — it currently takes eight years — and inflation and cost-of-living adjustments for pensions and 401(k) retirement plans.

But then the issue of moving work from Mexico to United States plants arose as a major stumbling block.

G.M. has three vehicle-assembly plants in Mexico that make S.U.V.s and pickup trucks sold under the Chevrolet and GMC brands. The company’s decision a few years ago to make the new Chevrolet Blazer in Mexico in particular has rankled the union since the midsize S.U.V. is the kind of vehicle that typically can be made profitably in United States plants.

At the same time, G.M. has closed a car plant in Lordstown, Ohio, and engine and transmission plants in Baltimore and in Warren, Mich. A second car plant in Detroit is scheduled to close in January.