Stellantis NV is suing the United Auto Workers to prevent a possible strike stemming from a dispute about promised investments at its U.S. assembly plants.

The United Auto Workers will ask members to greenlight a strike against Stellantis NV, escalating an already tense standoff with one of the world’s largest car manufacturers.

The United Auto Workers gave union leaders permission to call a strike against the biggest U.S. carmakers if they fail to agree on a new contract before Sept. 15, a procedural formality that sets the stage for three weeks of tough negotiations.