DETROIT — Stellantis is suing the United Auto Workers, escalating a monthslong battle between the trans-Atlantic automaker and American union, CNBC has learned.

In an internal message Friday to employees that was confirmed to be authentic, the company said it is suing the UAW as well as a local chapter in California that participated in a strike authorization request vote at Stellantis’ Los Angeles Parts Distribution Center.

“This lawsuit would hold both the International and the local union liable for the revenue loss and other damages resulting from lost production due to an unlawful strike,” Tobin Williams, Stellantis senior vice president of North America human resources, said in the message.

A supermajority of UAW members at Stellantis’ Los Angeles Parts Distribution Center voted to request strike authorization from the International Executive Board if the company and union can’t reconcile, the union said Friday morning.

The complaint is intended to “prevent and/or remedy a breach of contract” by the UAW, according to a copy of the lawsuit that was filed Thursday in U.S. District Court in the Central District of California.

The lawsuit argues that if the union does strike, the court “should award Stellantis monetary damages” that result from a breach of contract.

UAW President Shawn Fain addressed the lawsuit Friday in a letter to union leadership at Stellantis. He called it and other actions by the company “desperate actions from a desperate executive who has lost control.”

“Our legal team has complete confidence in our right to strike. The company’s legal threats are just that—threats intended to intimidate us, so we won’t fight back,” Fain said.

The dispute between the two sides centers on the union alleging Stellantis has not kept contractual obligations as part of a deal the two sides reached late last year. It comes after Stellantis has made several cuts to plant production, conducted worker layoffs and delayed potential investments outlined as part of the 2023 contract.

Fain has routinely said the union will strike if needed, however Stellantis has argued that would be unlawful under the contract.

The automaker has contended that there’s language in the contract that gives it leniency to change plans based on market conditions, plant performance and other factors.

The company reiterated that stance in its lawsuit and cited “Letter 311,” which includes the company’s expected investments: “The planned future investments in the letter are conditional, require Company approval, and are subject to change based on these business factor contingencies.”

The lawsuit came the same day Fain and union members held their latest rally against Stellantis in suburban Detroit.

“We’re here today for one reason. Stellantis CEO Carlos Tavares is out of control and it’s once again up to UAW members to save this company from itself,” Fain said during the event. “A strike will cripple this company. And if we have to strike, it’s Stellantis’ decision to do so because they are not honoring their commitment.”

The union and several local chapters have filed grievances against the automaker regarding contract obligations and other issues.

Stellantis, in the lawsuit, called the grievances a sham designed to “justify mid-contract strikes against Stellantis that otherwise would violate the [contract’s] no strike clause.”