United Auto Workers President Shawn Fain was adamant last week that the Big Three automakers can still offer more despite already putting record contracts on the table.

On Monday morning, he underscored that message, calling on 6,800 autoworkers at Stellantis' Sterling Heights Assembly Plant outside Detroit to walk out.

The plant, Stellantis' largest, makes the bestselling RAM 1500 trucks.

The expansion, announced by UAW in a statement, brings the total number of autoworkers on strike to over 40,000, with thousands more laid off in what the auto companies call a ripple effect of the strike.

The union said it took the step because "Stellantis lags behind both Ford and General Motors in addressing the demands of their UAW workforce."

"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," the UAW said.

Record contracts — but not enough

Fain had already warned in a Facebook Live address on Friday that Stellantis' offer was lagging Ford and GM.

All three automakers have proposed wage raises of 23% over the life of the contract, a significant increase from their initial proposals.

Stellantis has also said it would increase its 401(k) retirement contributions to 9.5%, among other improvements, but has not agreed to a return of traditional pensions.

Stellantis had agreed to cut the number of years it takes permanent employees to reach the top wage from eight to four, but Ford has now reduced the time to three years, Fain said.

By laying out the Big 3's proposals side-by-side, Fain is making clear that he expects all three to get on the same page and will amp up the pressure on companies to get them to comply.

"We're going to hit when we need to hit and where we need to hit," he said on Friday.

Responding to Fain's Facebook live remarks, Stellantis released a statement saying talks continued to be productive.

"We have made progress on narrowing the gaps on significant issues that will bring immediate financial gains and job security for our employees. Our focus remains on resolving those issues as soon as possible and finding solutions that protect the Company and our employees," the statement read.