Movement conservatives and self-styled Republican populists have long been joined in conflict with the party establishment, but the GOP presidential race has set them on a collision course. Friction between these two GOP factions is being fueled by the increasingly bitter battle between Sen. Ted Cruz of Texas and New York celebrity businessman Donald Trump in the race for the Republican presidential nomination. Cruz represents ideologically committed conservatives, although he has appealed with some success to populist voters. Trump has given big government populists, who have aligned with the GOP mostly for cultural reasons, a champion, while appealing to some...