STERLING HEIGHTS, Mich. — Thousands of workers at a Stellantis plant in Michigan joined the United Auto Workers in walking off the job on Monday, nearly six weeks after the union launched its strike against Detroit’s Big Three automakers.

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Union officials announced that 6,800 workers at Stellantis’ Sterling Heights Assembly Plant joined the strike on Monday. Stellantis’ best-selling Ram 1500 pickup trucks are made at the facility.

The surprise walkouts came after UAW President Shawn Fain said that offers so far floated by Stellantis, General Motors and Ford have fallen short. He acknowledged Friday, however, that workers were being offered “record contracts.”

“These are already record contracts, but they come at the end of decades of record decline, so it’s not enough to be the best ever when autoworkers have gone backwards over the last two decades,” he said. “That’s a very low bar.”

He praised striking workers for holding the line amid ongoing uncertainty about their jobs.

“Our ability to hold out, to hit the companies economically and to withhold our labor — this is our leverage and this is our path to victory,” he said. “We have one tool, and that’s solidarity.”

The walkouts announced Monday brought the total number of workers striking over 40,000, according to UAW.

Union workers walked off the job at three GM, Stellantis and Ford plants on Sept. 15, kicking off the strike. Employees at dozens more plants have since joined the walkouts.

Union officials are pushing to get employees more pay and better benefits. Automakers have argued that they can’t afford to meet UAW’s demands.