The new walkout brings the total number of striking workers to more than 40,000 and continues the UAW’s strategy of ramping up pressure on the automakers by keeping the companies guessing where the next work stoppage might be as they negotiate a new contract with the union.

UAW President Shawn Fain has been expanding the strike on a piecemeal basis since the it began Sept. 15.

The news comes nearly two weeks after the union’s last strike expansion, when thousands of UAW members unexpectedly walked out of a highly profitable Ford plant in Kentucky on Oct. 11. Monday’s strike expansion mirrors the one at Ford in that it targets a lucrative truck plant.

“We escalate if and only if we think it will win big,” Fain said Friday on a Facebook Live address.

The walkout on the Stellantis plant, which is located in the politically crucial Macomb County and produces RAM 1500 trucks, comes days after Stellantis on Friday said talks with the union were “productive, building on the momentum from the past several weeks.”

With Fain repeatedly rejecting the automakers’ offers as inadequate, some executives have been speaking more publicly about their frustrations with the talks.

Stellantis in a statement Monday afternoon said it was “outraged” at the Sterling Heights escalation, adding that the strike strategy could do long-term damage to the company and the market share of union-represented auto companies.

The company said it’s been waiting for a counter-proposal since its last offer to the union on Thursday.

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

Last week, Ford Executive Chair Bill Ford called the negotiations “acrimonious.”

And on Thursday, the head of global manufacturing for GM said the company couldn’t meet all of the UAW’s demands without “devastating” it.

Nonetheless, GM came back to the union with some sweeteners on Friday: It said the majority of its workforce would make $40.39 per hour under the offer.

It’s been nearly six weeks since the first group of workers went on strike. Fain on Friday told workers to hold steady on the picket lines.

“Every time they make an offer, it’s the best they can do, it’s a record offer. And then two days later, there’s a new record,” Fain said. “What that should tell you is there’s room to move.”

All three companies are currently proposing 23 percent wage increases.

The companies have also been warning about possibly broader economic consequences the longer the strike drags on.

All three have laid off workers during the strike, citing downstream effects of work stoppages at striking plants. Ford has laid off the most, according to counts provided by the companies, at 3,167 as of Sunday. Stellantis laid off an additional 100 employees on Thursday, bringing its total to 1,520, according to the company.

As of Thursday, GM had sent about 2,350 workers home in addition to those on strike, the company said. It also pointed to thousands more employees laid off across supplier facilities.

“If it continues, it will have a major impact on the American economy and devastate local communities,” Ford said. “The supply base is very fragile and will start collapsing with an expanded strike.”

But the union and its supporters have said any short-term damage would be more than made up for with a contract that provides good wages and benefits, which will help stoke local economies.

Toby Eckert and Nick Niedzwiadek contributed to this report.