DETROIT (AP) — The United Auto Workers union has widened its strike against General Motors, the lone holdout among the three Detroit automakers, after reaching a tentative contract agreement with Jeep maker Stellantis.

The escalated walkout began Saturday evening at a Spring Hill, Tennessee plant, GM’s largest in North America, just hours after the Stellantic deal was reached. Its nearly 4,000 workers join about 18,000 already striking at GM factories in Texas, Michigan and Missouri and Tennessee.

The UAW did not immediately explain what prompted the new action after 44 days of targeted strikes. The added pressure on GM is substantial as Spring Hill makes engines for other plants in addition to big-ticket vehicles including the electric Cadillac Lyriq, GMC Acadia and Cadillac crossover SUVs.

“The Spring Hill walkout affects so much of GM’s production that the company is likely to settle quickly or close down most production,” said Erik Gordon, a University of Michigan business professor. The union wants to wrap negotiations with all three automakers so “Ford and Stellantis workers don’t vote down (their) tentative agreements because they want to see what GM workers get.”

The Stellantis deal mirrors one reached last week with Ford, and saves jobs at a factory in Belvidere, Illinois, that Stellantis had planned to close, the UAW said.

GM said it was disappointed with the additional strike at the Spring Hill plant, which has 11 million square feet of building space, “in light of the progress we have made.” It said in a statement that is has bargained in good faith and wants a deal as soon as possible.

In a statement, UAW President Shawn Fain lamented what he called “GM’s unnecessary and irresponsible refusal to come to a fair agreement.”

“Everybody’s really fired up and excited,” Spring Hill assembly line worker Larry Montgomery said by phone on Sunday. He said workers were taken by surprise by the strike call. “We thought it was going to happen earlier.”

UAW Local 1853 President John Rutherford in Spring Hill didn’t immediately return a telephone message.

Fain said in a video appearance Saturday night that 43,000 members at Stelantis would have to vote on the deal — just as Ford workers must. About 14,000 UAW workers had been on strike at two Stellantis assembly plants in Michigan and Ohio, and several parts distribution centers across the country. The company makes Jeep and Ram vehicles.

The pact includes 25% in general wage increases over the next 4 1/2 years for top assembly plant workers, with 11% coming once the deal is ratified. Workers also will get cost-of-living pay that would bring the raises to a compounded 33%, with top assembly plant workers making more than $42 per hour. At Stellantis, top-scale workers now make around $31 per hour.

Like the Ford contract, the Stellantis deal would run through April 30, 2028.

Under the deal, the union said it saved jobs in Belvidere as well at an engine plant in Trenton, Michigan, and a machining factory in Toledo, Ohio.

“We have reopened an assembly plant that was closed,” Fain said. The deal includes a commitment by Stellantis to build a new midsize combustion-engine truck at the Belvidere factory that was slated to be closed. About 1,200 workers will be hired back, plus another 1,000 workers will be added for a new electric vehicle battery plant, the union said.

Vice President Rich Boyer, who led the Stellantis talks, said the workforce will be doubled at the Toledo, Ohio, machining plant. The union, he said, won $19 billion worth of investment across the U.S.

Fain said Stellantis had proposed cutting 5,000 U.S. jobs, but the union’s strike changed that to adding 5,000 jobs by the end of the contract.

Gordon, the University of Michigan professor, said the Stellantis deal “shows that the car companies feel they are at the mercy of the UAW, that the UAW is not going to give any mercy, and that companies will be co-governed by their boards and the UAW.”

He said competing companies with non-unionized workforces, which include Toyota and Tesla, “couldn’t have gotten a better year-end gift.”

Under the Stellantis contract, a top-scale assembly plant worker’s base wage will exceed all increases in the past 22 years. Starting wages for new hires will rise 67% including cost-of-living adjustments to over $30 per hour, it said in a statement. Temporary workers will get raises of more than 165%, while workers at parts centers will get an immediate 76% increase if the contract is ratified.

Like the Ford agreement, it will take just three years for new workers to get to the top of the assembly pay scale, the union said.

The union also won the right to strike over plant closures at Stellantis, and can strike if the company doesn’t meet product and investment commitments, Fain said.

Bruce Baumhower, president of the local union at a large Stellantis Jeep factory in Toledo, Ohio, that has been on strike since September, said he expects workers will vote to approve the deal because of the pay raises above 30% and a large 11% raise immediately. “It’s a historic agreement as far as I’m concerned.”

Some union members had complained that Fain promised 40% raises to match what he said was given to company CEOs, but Baumhower said that was merely an opening bid.

“Ultimately, the numbers they did come to agree with is what the UAW wanted,” said Jermaine Antwine, a 48-year-old Stellantis worker who had been picketing the automaker’s Sterling Heights, Michigan, plant Saturday. A team leader in materials at the plant, the Pontiac, Michigan man has has 24 years with the automaker.

Negotiations between the UAW and Stellantis had intensified Thursday, the day after the Ford deal was announced.

The union began targeted strikes against all three automakers on Sept. 15 after its contracts with the companies expired. At the peak, about 46,000 workers were on strike against all three companies, about one-third of the union’s 146,000 members at the Detroit three.

With the Ford deal, which set a template for the other two companies, workers with pensions will see small increases when they retire, and those hired after 2007 with 401(k) plans will get large increases.

Other union leaders who followed aggressive bargaining strategies in recent months have also secured pay hikes and other benefits for their members. Last month, the union representing Hollywood writers called off a nearly five-month strike after scoring some wins in compensation, length of employment and other areas.

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Bajak reported from Boston. AP writers John Raby in Charleston, West Virginia, Corey Williams in Sterling Heights, Michigan, and Haleluya Hadero in Jersey City, New Jersey, contributed to this report.