DETROIT -- First it was Ford, then Stellantis, and now a General Motors factory has been added to the growing list of highly profitable plants where the United Auto Workers union is on strike.

On Tuesday, about 5,000 workers walked out at GM's factory in Arlington, Texas, that makes big, high margin SUVs such as the Chevrolet Tahoe and Cadillac Escalade.

The strikes in Texas, as well as at the largest Ford factory in the world in Louisville, Ky., and a Stellantis plant that makes lucrative Ram pickups in Michigan, are aimed at getting the companies to capitulate to union demands for richer wages and benefits than the automakers so far have offered.

But judging from statements out of Detroit, the companies are at or near the limit on how much they're willing to budge to end a series of targeted strikes now involving 46,000 workers that began on Sept. 15. About 32 per cent of the union's 146,000 members at the companies are on strike, and the automakers are laying off workers at other plants as parts shortages cascade through their systems.

In announcing the Arlington strike, UAW president Shawn Fain noted that GM posted big earnings on Tuesday, yet its offer to the union lags behind Ford, preserving a two-tier wage structure and offering the weakest 401(k) contribution of all three automakers.

"It's time GM workers, and the whole working class, get their fair share," Fain said.

But GM CEO Mary Barra told investors on the company's earnings conference call that the automaker already has made a record offer and won't sign a contract that jeopardizes the company's future.

"We will not agree to a contract that isn't responsible for our employees and for our shareholders," she said. "We need to make sure we have a contract that is going to allow us to compete and win in what is a challenging market for EVs and also allows us to support the business that we have with strong margins in our (internal combustion engine) business."

Last week, Ford told reporters that it had reached the limit of what it was willing to pay to end the nearly six-week-old strike, bringing out executive chairman Bill Ford to urge strikers to return to work. On Monday, after the union took down the pickup plant in Sterling Heights, Mich., north of Detroit, Stellantis said it was "outraged" by the escalation because it improved its offer to include a 23 per cent wage increase over four years.

All three automakers have said they won't stick themselves with high labour costs that would make their vehicles more costly than non-union competitors.

Talks continued Tuesday with Stellantis and Ford, with new offers from the union either coming or delivered at both companies. The status of talks with GM wasn't clear.

Early on, the union struck at plants that didn't make the companies' most expensive and profitable vehicles. But as the strikes dragged on, Fain has targeted truck and SUV plants in an effort to empty the companies' wallets.

At the same time, workers are getting by on US$500 per week of strike pay, hardly enough to pay the monthly bills. The payments also are making a dent in the union's strike fund, which was US$825 million when the strikes began. Fain said it was still healthy.

Thomas Kochan, a professor of work and employment at the Massachusetts Institute of Technology, said adding the GM SUV plant means the negotiations are at a pivotal point.

"The pressures for reaching an agreement that everybody can live with are immense on both the company and the union," he said. "The effects of an expanded strike across the three companies and prolonged over time would be profound, and would have very serious negative effects on the companies and on the workforce."

The companies, he said, are close to the limits on their offers and the union is close to what it legitimately can expect to get.

"There comes a time where the parties have to have very private conversations in negotiations," Kochan said. "It's time for the public rhetoric to stop."

On the picket line in Texas, Ethan Pierce, a material handler with more than 23 years at GM, said workers sacrificed, making concessions to help save GM when it was in dire financial trouble around the 2008 financial crisis. "We started asking for some of our stuff back. They didn't want to give it to us," Pierce said.

Now, with inflation driving up prices, workers are struggling, he said. Among the sticking points is GM's refusal to let workers go on strike over plans to close factories, Pierce said.

"If you're being treated unfairly, sooner or later you have to stand up," he said. "When we get treated better, everybody else gets treated better."

The addition of the Arlington plant came just after GM announced strong third-quarter financial results. The SUVs are among GM's most profitable vehicles.

The company on Tuesday posted a net profit of just over US$3 billion for the quarter, down seven per cent from a year ago. But the company reported strong demand and prices for its vehicles.

GM later said that it's disappointed in the escalation at Arlington, calling the strike "unnecessary and irresponsible" and said it will have negative ripple effects on dealers, suppliers and communities.

Because the striking plants supply or get parts from other factories, the automakers say they've been forced to lay off another 7,672 workers. And shares of General Motors Co. are down more than 14 per cent this year, touching lows Tuesday that haven't been seen since 2020 during the pandemic, when the company's sales growth tumbled almost 11 per cent.

Last week GM made an offer that increased its previous one by about 25 per cent in total value, the company said.

Barra said GM has made a record offer to the union that will raise top factory pay to US$40.39 per hour, or roughly US$84,000 per year in four years.

The company also said the strike is expected to cut pretax earnings by US$800 million this year, and another US$200 million per week after that. And those estimates were made prior to the Arlington strike, GM said.

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AP Chief Photographer for Texas Julio Cortez contributed to this report from Arlington, Texas, and AP Business Writer David Koenig contributed from Dallas.