On Friday, General Motors put forward a more lucrative contract proposal. By calling for the strike at the Sterling Heights plant, the U.A.W. is trying to pressure Stellantis into at least matching the terms that G.M. offered.

“Stellantis has the worst proposal on the table regarding wage progression, temporary worker pay and conversion to full-time, cost-of-living adjustments, and more,” the U.A.W. said in a statement on Monday.

In its offer, G.M. proposed raising workers’ wages by 23 percent over four years. That would lift the wage for all full-time workers from $32 an hour to more than $40, giving them a base pay of about $84,000 a year, not including overtime or profit-sharing bonuses.

The walkout at the Ram plant is the first escalation in the strike since the U.A.W. called 8,700 workers to leave their jobs at Ford’s largest plant, in Louisville, Ky., on Oct. 11. That plant produces the Super Duty version of the popular F-150 pickup truck and the Ford Expedition, a full-size sport utility vehicle.

In a statement, Stellantis said the company was “outraged” by the expansion of the strike, noting that it made a comprehensive new proposal to the union on Thursday morning and was waiting for a counterproposal from the U.A.W.