(Bloomberg) -- The United Auto Workers expanded its strike against General Motors Co. just hours after reaching a tentative agreement on a new contract with Stellantis NV.

Union workers walked out of GM plants in Spring Hill, Tennessee, on Saturday evening as negotiations with the automaker faltered. An assembly plant there makes mid-size SUVs and the Cadillac Lyriq electric vehicle, along with engine and metal-stamping operations.

More than six weeks after the UAW began its historic strikes against Detroit’s legacy automakers, GM is now the only one of the companies that doesn’t have a deal with the union. The walkouts have cost the industry billions of dollars.

Ford Motor Co. was first to reach a tentative agreement on Oct. 25. Stellantis followed on Saturday after making concessions on job security. Those agreements still need to be voted on by the companies’ union members.

Read More: Stellantis, UAW Reach Tentative Deal to End Six-Week Strike

Like Stellantis and Ford, GM has agreed to a 25% wage increase for hourly workers along with cost-of-living allowances, according to people familiar with the negotiations who weren’t authorized to speak publicly. But negotiations at GM have been held up by disagreements over issues including temp workers, the people said.

GM has more temporary workers than Ford, as does Stellantis, making the issue a thorny one to settle at the bargaining table. GM has as many as 10% temporary workers at any given time, and it would be costly to give them the 150% or more raises that have been offered by Ford and Stellantis.

“We are disappointed by the UAW’s action in light of the progress we have made,” GM said in an emailed statement. “We have continued to bargain in good faith with the UAW, and our goal remains to reach an agreement as quickly as possible.”

Spring Hill Walkout

At Spring Hill, a weekend crew of more than 70 workers walked out at 5 p.m. local time, according to Jason Spain, shop committee chairman of Local 1853. There are about 3,200 union workers at Spring Hill, Spain said, and UAW President Shawn Fain told the union’s regional head on Saturday that they needed to go on strike.

“He said GM isn’t budging,” Spain said in a phone interview. “I’m not sure what they’re hung up on.”

The Spring Hill assembly plant, once home to Saturn vehicles, wasn’t producing cars over the weekend, Spain said. That gives GM the rest of the weekend to come to a deal before the company would lose production, assuming the union sends members back to work once they have a tentative agreement, as it has with Ford and Stellantis.

Before Ford and Stellantis reached their deals, the UAW strike had grown to include more than 45,000 workers at eight assembly plants and 38 parts-distribution facilities. Spring Hill is the ninth assembly plant to be struck.

Underperforming Stocks

GM and Ford have seen steep declines since July and are underperforming the S&P 500 this year. Stellantis is an outlier with its US shares up 27% so far in 2023.

Ford’s deal this week put pressure on its Detroit rivals to wrap up their negotiations and get back to work. Both GM and Ford both pulled earnings guidance after the strike muddied their outlook. Ford said Oct. 26 that the work stoppage cost the company $1.3 billion. Earlier in the week, GM said its strike costs had reached $800 million and will cost at least $200 million a week.

This was the first time all three of Detroit’s legacy automakers were struck at the same time, a bet by the UAW’s Fain to keep them guessing by incrementally adding more plants to the work stoppage. His strategy is paying off with record wage hikes and the restoration of some benefits that the union forfeited during the financial crisis more than a decade ago.

The gains come amid a resurgence of labor activism in the US. Emboldened by tight labor markets and agitated by inflation and risks shouldered during the pandemic, unionized workers have notched a series of victories in the last year at prominent US companies, including Kaiser Permanente and United Parcel Service Inc.

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