Harrison McClary / Reuters Workers assemble engines as they make their way through the assembly line at the General Motors manufacturing plant in Spring Hill, Tennessee, on Aug. 22.

The United Auto Workers called for a strike against General Motors starting Monday after the union and the automaker failed to settle on a new four-year collective bargaining agreement.

If the two sides can’t agree on a contract by midnight, nearly 50,000 UAW members won’t be showing up to work, likely grinding production to a halt at plants around the country. It would be the first strike by auto workers at GM in more than a decade.

The workers’ contract expired at midnight Saturday night. Union officials from UAW locals everywhere met in Detroit Sunday morning to discuss next steps. The union soon put out a statement saying workers had “helped rebuild General Motors when they were near extinction, now they’ve reached record level profits.”

“If GM refuses to give even an inch to help hard-working UAW members and their families then we’ll see them on the picket lines tonight,” the union said.

GM said in a statement that it had bargained in good faith and “with a sense of urgency,” but it was “disappointed” to see the UAW had called for a strike.

“We presented a strong offer that improves wages, benefits and grows U.S. jobs in substantive ways,” the company said.

As HuffPost previously reported, this round of contract talks was expected to be the most contentious in many years. The Big Three ― GM, Ford and Fiat Chrysler ― are trying to keep costs down with warnings of a sales slowdown, but their robust profits over the last four years have left workers with little reason to give anything up.

As one worker said of GM earlier this week, “It’s time for them to give back.”

The three companies and the UAW engage in “pattern” bargaining: The union settles a contract with one company first, setting a model for the two contracts that follow. This time, the union chose to negotiate with GM first, probably because it’s been the most profitable. The UAW has already agreed to extend its earlier contracts with Ford and Fiat Chrysler, leaving no imminent threat of a strike at those companies.

GM said its offer to the union included “solutions for unallocated assembly plants” in Michigan and Ohio ― i.e., the company probably agreed to steer production to certain plants slated to be idled. Getting guarantees on production has been a priority for the union since the plant idlings GM announced late last year have left workers laid off or having to relocate to keep their jobs. But it isn’t clear whether the company’s offer on allocation met the UAW’s expectations.