The heated rhetoric and large candidate field have made the Republican primary race look loud and messy. But here's a simple way to view the landscape: The GOP electorate is divided into three, broad groups, each roughly equal in size. There is the socially conservative wing, which has elevated Sen. Ted Cruz and Ben Carson. The GOP's establishment-friendly, more-centrist wing backs Sen. Marco Rubio and a few others. And then there are the Donald Trump voters, a largely working-class and male group drawn together by Mr. Trump's unique persona and, in large measure, his promise to end illegal immigration. With...