One night last month, on the recommendation of a man known online as Captain K, a small group gathered in an Arizona parking lot and waited in folding chairs, hoping to catch the people they believed were trying to destroy American democracy by submitting fake early voting ballots. Captain K — which is what Seth Keshel, a former U.S. Army intelligence officer who espouses voting fraud conspiracy theories, calls himself — had set the plan in motion. In July, as states like Arizona were preparing for their primary elections, he posted a proposal on the messaging app Telegram: “All-night patriot...