Senator Ted Cruz of Texas got an early start to the 2016 presidential campaign in March by being the first of what would become a vast field to announce his candidacy. Now, he has turned that starting advantage into a sprawling ground game. For every county in the first four voting states of Iowa, New Hampshire, South Carolina and Nevada, the Cruz campaign has locked down county chairs in charge of not just lending their names to the campaign, but of spearheading outreach and organizing efforts. “The only way we will turn our country around is through a grass-roots uprising...