Stellantis said it resumed negotiations with the union Monday and described the talks as “constructive.” A spokesman for General Motors said representatives of the company and the United Auto Workers also were continuing to negotiate.

However, UAW President Shawn Fain said on NPR, “We have a long way to go,” and if the companies don’t respond to the union’s demands, “then we will escalate action.”

In a video statement late Monday, Fain said more factories could be targeted if “serious progress” toward an agreement isn’t reached by Friday at noon.

“We’re not messing around,” he said.

So far the strike is limited to about 13,000 workers at three factories — one each at GM, Ford and Stellantis, the successor to Fiat Chrysler.

However, the union’s strategy hinges on its ability to escalate the strike quickly, and the carmakers are warning of potential layoffs as the limited strike reduces the amount of material needed at plants that remain open.

GM said Monday that 2,000 UAW-represented workers at an assembly plant in Kansas City are “expected to be idled as soon as early this week” because of a shortage of supplies from a GM plant near St. Louis, where workers walked off the job Friday.

Workers at the Kansas City plant build the Chevrolet Malibu and Cadillac XT4.

The strike could begin to affect suppliers and their employees too. CIE Newcor told Michigan officials that it expects a one-month closure of four plants in the state to start Oct. 2 and idle nearly 300 workers.

In a sign of concern of the strike’s potential economic and political fallout, the Biden administration stepped up its response.

Treasury Secretary Janet Yellen said she is hoping for a quick resolution, while adding that is too soon to gauge the strike’s impact.

“It’s premature to be making forecasts about what it means for the economy. It would depend on how long the strike lasts and who would be affected by it,” she said on CNBC.

Yellen said labor activism this year — strikes by Hollywood writers and actors, by workers at about 150 Starbucks locations and walkouts that were narrowly averted at United Parcel Service and West Coast ports — has been driven by a strong labor market and high demand for workers.

President Joe Biden is sending two top administration officials to Detroit to meet with both sides. Biden has backed the UAW in brief public comments, saying that the automakers have not fairly shared their record profits with workers.

An administration official said Monday that acting Labor Secretary Julie Su and senior aide Gene Sperling will not serve as mediators — they won’t be at the bargaining table — but are going to Detroit “to help support the negotiations in any way the parties feel is constructive.” The official was not authorized to discuss private discussions and spoke anonymously.

Fain said the Biden administration won’t broker a deal

“This is our battle. Our members are out there manning the picket lines,” he said Monday on MSNBC. “This battle is not about the president, it’s not about the former president” — a reference to reports that former President Donald Trump plans to skip a debate for Republican presidential candidates next week to meet with striking autoworkers in Detroit.

A key feature of the UAW strategy is the threat of escalating the strike if the union is unhappy with the pace of bargaining.

On Monday, Ford workers on a picket line outside a plant in the Detroit suburb of Wayne were joined by members of other unions and the occasional politician.

Tevita Uhatafe, an aircraft-maintenance worker from Arlington, Texas, showed his support and saw what it might look like if UAW members strike against a GM truck plant in his hometown.

“This is a fight that is most likely going to happen in our backyard,” Uhatafe said.

U.S. Rep. Haley Stevens, D-Mich., said she walked the picket line because the strike “is showcasing a modern movement for worker justice and worker fairness.”