The union says that 6,800 members walked out Monday morning and shut down the Sterling Heights, Michigan, Assembly Plant, a huge profit center for the company.

The newest strike action comes just three days after union President Shawn Fain reported progress in talks with General Motors and Stellantis but said the companies will have to make better offers. No progress was reported with Ford, which last week said it had the best offer of the three.

The union went on strike Sept. 15 at one assembly plant from each company. About 41,000 workers are now on strike against all three automakers. The strikes, now in their sixth week, cover seven assembly plants and 38 parts warehouses. About 28% of the union’s work force at the three companies are now on strike.

General Motors, which increased its offer last week, and Ford were spared in the latest escalation. At first the union avoided striking at pickup and large SUV plants, which at all three produce vehicles that make the most money for the companies. But that changed two weeks ago when the UAW took out a giant Ford heavy-duty pickup and SUV plant in Louisville, Kentucky.

In previous strikes the union has targeted a particular company and reached an agreement that served as a pattern for a deal with the other two.

In a statement, the union said that offers from Stellantis, formed in the 2021 merger of Fiat Chrysler and France’s PSA Groupe, lag Ford and GM despite the automaker having the most revenue and highest profits of the three.

Stellantis has the worst offer on cost of living raises, how fast workers move to the top of the pay scale, temporary worker pay, converting temps to full time, and other issues, the union said.

In a statement, Stellantis said it was “outraged” that the Sterling Heights plant was added to the strike list because the company improved its offer on Thursday but has not received a response from the UAW. The new offer included a 23% wage increase over four years, nearly a 50% increase in retirement savings contributions and added job security provisions, the company said.

“Our very strong offer would address member demands and provide immediate financial gains for our employees,” the statement said. “Instead the UAW has decided to cause further harm to the entire automotive industry as well as our local, state and national economies.”

With every plant it adds, the union “sacrifices domestic market share to nonunion competition,” hurting the company’s ability to invest and compete, the statement said.

The union expects to make counteroffers to Stellantis and GM soon.

DeSean McKinley, 45, of Detroit, who has worked at the Sterling Heights plant for nine years, was surprised by the strike but said he supports the union’s action.

“Now, we’re here, fighting for solidarity — fighting for what’s right for us,” McKinley said. “We’re not going to keep playing. So we’re going to shut down your bread and butter.”

By taking down the Stellantis factory, the union is signaling Ford and GM to improve their offers, Wedbush analyst Dan Ives said. Last week it appeared a deal might be in the works, but Fain said Friday that the companies will need to pay more.

“It takes a potential deal that appeared on the table — at least for now — off of it,” Ives said, predicting the the union will announce new strike locations later this week. “There could be some tough talks ahead,” Ives said.

On Friday, Fain said Stellantis and GM have made wage offers that matched Ford’s 23% over the life of a four year contract. But, speaking in his characteristic sharp tones, the union president insisted that the companies can go further.

“We’ve got cards left to play, and they’ve got money left to spend,” Fain said.

While Fain said the companies keep touting that they’ve made record offers to the UAW, he said they’re insufficient to make up for how much ground workers have lost during the past two decades. Each time the automakers make an offer, Fain said, they insist it’s the best they can do, only to return days later with a better offer.

“What that should tell you,” Fain said, is that “there’s room to move.”