In the last five years, as American consumers flocked to high-margin trucks and sport utility vehicles, both G.M. and its unionized work force have prospered. In the last three years, G.M. has made $35 billion in profit in North America, and workers have been given profit-sharing checks averaging $11,000 a year.

But even though G.M. was scaled down in bankruptcy, it still has excess production capacity. The company has enough plants to make about one million more vehicles than it is selling, according to the Center for Automotive Research, an independent, nonprofit group.

At the same time, G.M. is spending heavily to develop electric vehicles and self-driving technology, and its business outlook is uncertain. Auto sales have slowed in the United States, and some analysts expect a substantial decline in new-vehicle sales in 2020. Sales in China, the world’s largest auto market, have also softened.

Mr. Wakefield said automakers were expected to spend some $225 billion over the next five years on development of electric and self-driving vehicles. “Industry profit is still good, but it’s down from its peak of a few years ago,” he said. The combination of heavy spending and slowing sales “has created some problems for them.”

That’s one reason that G.M. has been eager to retain flexibility in the size and deployment of its work force. Among the issues in dispute was the extent of G.M.’s use of temporary workers — now 7 percent of its head count — and their path to full-time status.

Marisol Gonzalez-Bowers worked for G.M. at Lordstown for about 24 years, most recently in “materials,” transporting parts to the assembly line. Relocating after the shutdown, she recently took a job at the company’s Lansing Delta Township plant in Michigan, where she was trained to assemble the insides of doors — the wires and plugs. Her trainers were almost all classified as temporary workers, she said, though they had been there four years.