The United Auto Workers expanded their ongoing strike Monday to Stellantis' Sterling Heights Assembly Plant, where 6,800 workers will join the picket line.

The SHAP is Stellantis’ largest plant and a big moneymaker where it builds the popular Ram light-duty pickup trucks. The UAW launched its targeted Stand Up Strike against the Detroit automakers on Sept. 15.

Monday's move was the fourth time the UAW has expanded the strike and comes after UAW President Shawn Fain detailed the latest proposals across the automakers on Friday, noting shortcomings in Stellantis' current offer. The areas where Stellantis lags its crosstown rivals, General Motors and Ford Motor Co., included cost-of-living adjustment, progression time to the top wage, profit sharing and wages for temporary employees, as well as offerings to retirees.

On Friday, Fain stood pat on the so-called Stand Up Strike, which is targeting certain plants across all three Detroit automakers. But he warned members to "be ready and stay ready to stand up" to take strike action at any time, saying the union still has "cards left to play" in terms of key plants it could take out at each automaker.

Outside of SHAP Monday, when Fain was asked about this latest move sending a message to GM, he replied, “Get ready, more is coming."

In a statement, Stellantis spokeswoman Jodi Tinson said the company is "outraged" that the UAW is expanding the strike against Stellantis" given that last Thursday, the automaker presented a new, improved offer to the UAW. She said the offer came after "multiple conversations that appeared to be productive, we left the bargaining table expecting a counter-proposal, but have been waiting for one ever since."

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Experts say striking the SHAP shows Fain squeezing harder to get a tentative agreement in place.

"Pickups are high-profit, high-sales vehicles, so shutting down the Ram 1500 assembly line will put a big dent in Stellantis' North American profits," said Erik Gordon, a labor expert and business professor at the Ross School of Business at the University of Michigan. "The quick change from Friday's no strike expansion to Monday morning's shutdown of an important plant shows that Fain's patience has gotten thinner than a potato chip."

Stellantis, which owns Chrysler, Dodge, Jeep, Ram and Fiat brands, produced more than 216,000 vehicles in the first eight months of this year, or 15.82% of its North American production, at the SHAP, said Marick Masters, labor expert and business professor at Wayne State University. So taking production down at this plant is a big hit to the automaker.

On Friday, Fain said in the most recent company proposals to the union, GM, Ford and Stellantis have each offered a 23% wage increase over the life of the contract and each has offered to phase out wage tiers. But proposal differences remained in the following areas:

• Progression period to the highest wage: Ford is down to three years; Stellantis, four years; GM is three years for all current employees and four years for future hires.
• Cost-of-living adjustment (COLA): Ford to reinstate cost-of-living to what it was in 2009. "At GM we are close with some tweaks left to make” and Stellantis has offered a “deficient COLA” formula that doesn’t kick in for the first year, Fain said.
• Profit-sharing: At all three companies the union “beat back concessionary” profit-sharing proposals, Fain said. At Ford, temps with 90 days on the job will be eligible to receive profit-sharing checks. GM has proposed including temps that have 1,000 hours on the job, at Stellantis “we’ve maintained, but haven’t won eligibility for temp workers” to receive profit-sharing yet.
• Temps: Ford and GM have raised the wage for temporary workers from $16.67 now to $21 an hour and improved the wait till permanent positions. At Stellantis the temp wage remains $20 an hour.
• Retirees: For current retirees, Ford is offering a $250 annual lump sum payment, GM is offering a one-time $1,000 lump sum payment, and Stellantis is not offering anything.

Contact Jamie L. LaReau: [email protected]. Follow her on Twitter @jlareauan. Read more on General Motors and sign up for our autos newsletter. Become a subscriber.