DETROIT — 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 several plants, the UAW said.

Presidents of the Ford union locals voted unanimously Sunday in Detroit to endorse that tentative contract after UAW President Shawn Fain explained its details, the union tweeted.

As he explained the particulars to the full membership in a later livestream, Fain, along with Chuck Browning, the UAW vice president, said the deal represents a “historical inflection point” for reviving union power in an America where “we were being left behind by an economy that only works for the billionaire class.”

“UAW members at Ford will receive more in straight general wage increases over the next 4 1/2 years than we have over the last 22 years combined,” Browning said.

Fain called the deal “a turning point in the class war that has been raging in this country for the past 40 years.”

The Ford and Stellantis pacts, which would run until April 30, 2028, include 25% in general wage increases for top assembly plant workers, with 11% coming once the deal is ratified.

The Ford agreement revives cost-of-living adjustments that the UAW agreed to suspend during in 2009 during the recession. And it ends “the abuse of temporary workers,” who not only will become permanent after nine months of continuous employment but will also get profit-sharing checks, Fain said.

He added the deal won’t force autoworkers to choose between “good jobs and green jobs” as the industry converts to electric vehicles: Workers at Ford’s electronic vehicle and battery plants will fall under UAW agreements once their workforce becomes majority union.

Talks With GM Continue

Meantime, the union continued talks Sunday with GM, said a spokesman for the automaker. No details were provided.

On Saturday, the company said in a statement it was disappointed with the expanded strike “in light of the progress we have made,” adding it 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.”

Fain said in a video appearance Saturday that 43,000 members at Stellantis 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.

At Stellantis, workers get cost-of-living pay that would bring raises to a compounded 33%, with top assembly plant workers making more than $42 per hour. Top-scale workers there now make around $31 per hour.

The union said the Stellantis deal saved jobs in Belvidere as well at an engine plant in Trenton, Michigan, and a machining factory in Toledo, Ohio. It also 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 double at the Toledo, Ohio, machining plant. The union, he said, won $19 billion worth of investment across the U.S.

That compares with more than $8 billion in investments that Browning said UAW won from Ford.

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.” 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, 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. Similarly, the union won the right to strike over plant closures.

Bruce Baumhower, president of the local union at a large Stellantis Jeep factory in Toledo, Ohio, that had been on strike since September, said he expected workers to vote to approve the deal because of pay raises including the immediate 11% raise on ratification. “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.

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 UAW workers were on strike — about one-third of the union’s 146,000 members at all three companies.

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. Workers also get two weeks parental leave — a first in UAW history, Fain said Sunday.

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.