PHOENIX — Inside a squat building ringed with a chainlink fence and concrete barriers in downtown Phoenix, election workers on Nov. 5 will begin a grindingly slow tally of every ballot cast in the vast expanse of stucco and saguaro that is Maricopa County, Arizona. In what has become the nation’s ultimate swing county, the count here could determine whether Democrat Kamala Harris or Republican Donald Trump will be the next U.S.