The United Auto Workers expanded its strike against General Motors Co. just hours after reaching a tentative agreement on a new contract with Stellantis NV.

Union workers walked out of GM plants in Spring Hill, Tennessee, on Saturday evening as negotiations with the automaker faltered. An assembly plant there makes mid-size SUVs and the Cadillac Lyriq electric vehicle, along with engine and metal-stamping operations.

More than six weeks after the UAW began its historic strikes against Detroit’s legacy automakers, GM is now the only one of the companies that doesn’t have a deal with the union. The walkouts have cost the industry billions of dollars.