By TOM KRISHER 

and FRANK BAJAK 

Associated Press

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 14,000 already striking at GM factories in Texas, Michigan and Missouri.

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 vehicles assembled in a total of nine plants as far afield as Mexico, including Silverado and Sierra pickups. One plant already on strike it supplies with engines, in Arlington Texas, makes full-size SUVs including the Tahoe and Suburban. Vehicles assembled at Spring Hill include 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.

On Sunday, presidents of the union locals involved voted to endorse their tentative contract with Ford after UAW President Shawn Fain explained it, said a union official who spoke on condition of anonymity because they were not authorized to make the information public. The vote count was not immediately known.

Fain was to address the full membership, which will now vote on the pact, on Facebook later Sunday.

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, 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 percent in general wage increases over the next 4 1/2 years for top assembly plant workers, with 11 percent coming once the deal is ratified. Workers also will get cost-of-living pay that would bring the raises to a compounded 33 percent, 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 percent including cost-of-living adjustments to over $30 per hour, it said in a statement. Temporary workers will get raises of more than 165 percent, while workers at parts centers will get an immediate 76 percent 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 percent and a large 11 percent raise immediately. “It’s a historic agreement as far as I’m concerned.”