WASHINGTON (AP) — Long before running for Congress, Marty O’Donnell was an award-winning music composer for the popular “Halo” video game franchise. His work drew legions of fans to an online forum, which the Nevada Republican has tried to turn into a galvanizing force as he tries to unseat a Democrat in a Las Vegas area district that could help decide House control. O’Donnell has served as the group’s de facto leader, referring to himself as its “benevolent dictator” while tapping its members for campaign cash and harnessing their enthusiasm in his bid. The campaign held a two-day event for...