The United Auto Workers delivered another surprise attack Monday in the union's ongoing strike against Detroit's Big Three automakers, this time targeting Stellantis' largest plant.

The UAW announced it is shutting down Stellantis' Sterling Heights Assembly Plant in Michigan, meaning the 6,800 union members who work at the facility are now off the job and joining the picket lines.

"Despite having the highest revenue, the highest profits (North American and global), the highest profit margins, and the most cash in reserve, Stellantis lags behind both Ford and General Motors in addressing the demands of their UAW workforce," the union said in a press release. "Currently, Stellantis has the worst proposal on the table regarding wage progression, temporary worker pay and conversion to full-time, cost-of-living adjustments (COLA), and more."

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The Sterling Heights facility produces RAM 1500 trucks, and its shutdown is a major blow to Stellantis, which owns more than a dozen auto brands, including Dodge, Chrysler and Jeep.

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FOX Business has reached out to Stellantis for comment.

The UAW launched its simultaneous strike against Ford, General Motors and Stellantis on Sept. 15, starting with one assembly plant at each of the Big Three. The union has steadily added further strike targets against all the automakers as contract negotiations have dragged on.

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The addition of the Sterling Heights plant brings the total number of UAW members on strike at the Big Three to more than 40,000 of the nearly 150,000 total across the automakers.