ARLINGTON, Va.—A few months ago, Max Moore and his allies were voices in the social media wilderness. Their campaign to “Make DC Square Again”—an effort to return Arlington County and the City of Alexandria to the District of Columbia—was getting likes and reposts on X amid a redistricting battle in the state. But social media isn’t quite reality. Things got a little more real on April 21, when Virginia voters greenlit a congressional map redrawn by their Democrat-dominated legislature. Many districts are anchored in slivers of deep-blue Northern Virginia, including Arlington and Alexandria, jeopardizing the reelection prospects of multiple House Republicans....