With the specter of political violence looming, the Department of Homeland Security has advised hundreds of communities on election safety. Luzerne County, Pa., is at the center of the unrest. With northeastern Pennsylvania awash again in the reds and oranges of a dazzling autumn, workers recently planted boulders around a government building in downtown Wilkes-Barre to address a seasonal ugliness. But this was no beautification project. Luzerne County is bracing for Election Day. Across the country, the doubts and anger ginned up by the spurious election-fraud claims of former President Donald J. Trump have unsettled the once-routine civic task of...