DETROIT — Faced with weakening sales, a deteriorating global economy and an unpredictable trade war, General Motors and striking auto workers appeared to be making progress Tuesday toward a four-year labor contract.

The two-day walkout by 49,000 workers brought to a standstill more than 50 factories and parts warehouses in the union’s first strike against the No. 1 U.S. automaker in over a decade. Workers left factories and formed picket lines at 12:01 a.m. Monday.

“They are talking, they’ve made progress,” said Brian Rothenberg, spokesman for the United Auto Workers union.

Talks continued into the evening. Bargainers could take a break during the night, but they planned to resume work Wednesday, Rothenberg said.

GM and the union are negotiating at a time of troubling uncertainty for the U.S. auto industry.

Driven up by the longest economic expansion in American history, auto sales appear to have peaked and are heading down. Long-term challenges also loom: GM and other carmakers are struggling to make the transition to electric vehicles.

Meanwhile, President Donald Trump’s trade war with China and his tariffs on imported steel and aluminum have raised costs for auto companies. A revamped North American free trade deal is stalled in Congress, raising doubts about the future of America’s trade in autos and auto parts with Canada and Mexico, which last year came to $257 billion.

“The trade picture has complicated things,” said Jonathan Smoke, chief economist at Cox Automotive.

One of the main sticking points is health care. GM is looking to cut its costs, but workers say they shouldn’t have to pay more because the company is making billions in profits. Union workers pay about 4 percent of their health care costs, but employees of large companies in the U.S. pay an average of 34 percent, according to the Kaiser Family Foundation.

Tina Black, who finishes up 10-speed automatic transmissions at a GM engine and transmission factory in the Detroit suburb of Romulus, said health insurance is the most important issue to her, and she doesn’t want GM to change anything.

The factory workers, she said, work 10 to 12 hours doing the same repetitive tasks, which can cause injuries.

“It breaks down your body,” said Black, of Garden City, Mich. “In order to take care of ourselves, we need that.”

On Sunday, GM said it offered pay raises and $7 billion worth of U.S. factory investments resulting in 5,400 new positions, a minority of which would be filled by existing employees. The company also said it offered higher profit sharing, “nationally leading” health benefits and an $8,000 payment to each worker upon ratification.

In addition, GM pledged to build a new all-electric pickup truck at a factory in Detroit and open an electric vehicle battery plant in Lordstown, Ohio, according to a person who spoke to The Associated Press on condition of anonymity.