Tens of thousands of General Motors workers officially went on strike at 11:59 p.m. ET Sunday after negotiations with the automaker broke down, the United Auto Workers union announced.Why it matters: The UAW's GM members are now participating in the nation's first auto strike in 12 years and the largest demonstration by any union against any U.S. business since UAW's last strike against GM in 2007. The latest: Terry Dittes, the union vice president, told AP negotiations would resume at 10 a.m. Monday, but the strike was still expected to go ahead.Context: UAW's GM members are steaming about the company's November 2018 decision to "unallocate" 5 plants in the U.S. and 1 plant in Canada. They demanded that the company agree to a new labor agreement by Sept. 14. The company broke the deadline Sunday morning. The strike affects about 46,000 hourly workers, per Axios' Joann Muller. Under the existing labor contract, GM can't close plants, except in the case of a sudden market downturn. However, the UAW sued GM for violating the agreement, saying the company was using semantics to idle plants.The UAW is demanding fairer wages, affordable health care, profits, job security and a path for permanent positions for temporary employees.The UAW hoped that a new labor contract for its GM members would set the standards for all labor contracts in the industry, benefitting its members who work at Ford and Fiat Chrysler.Yes, but: The union's position in negotiations was hit by a scandal involving misappropriation of union funds that implicates union president Gary Jones.Federal authorities accused Jones of helping current and former senior union officials embezzle member dues to buy personal luxuries, according to the Detroit News.What they're saying: Dittes told members in a letter after final negotiations on Saturday night that the union and company remained divided. He said the company would not receive an extension.The union vice president accused GM of refusing "to put hard working Americans ahead of their record profits of $35 billion in North America over the last three years."GM, the largest US automaker, said in a statement that it is still willing to work to reach a deal to build "a strong future for our employees and our business."President Trump tweeted, "Here we go again with General Motors and the United Auto Workers. Get together and make a deal!"Go deeper: GM to cut workers, idle U.S. manufacturing plants Editor's note: This article has been updated with President Trump's tweet.