TUSCALOOSA, Ala. — A decisive vote against the United Auto Workers union at two Mercedes factories in Alabama on Friday sidetracked the UAW’s grand plan to sign up workers at nonunion plants mainly in the South. But newly elected President Shawn Fain said the union will return to Mercedes and will press on with efforts to organize about 150,000 workers at more than a dozen auto factories across the nation. Employees at Mercedes battery and assembly plants near Tuscaloosa voted 56% against the union in an election run by the National Labor Relations Board. The vote count handed the union a serious setback a month after the UAW scored a breakthrough victory at Volkswagen’s 4,300-worker assembly factory in Chattanooga, Tennessee. The NLRB’s final tally showed a vote of 2,642 against the union, with 2,045 in favor.