Donald Trump doesn’t sound like most Republican candidates—and not just because he uses unusually pointed language. It’s also due to his policies. One prominent example: He’s talked about raising taxes on some wealthy Americans and corporations. The idea of raising taxes is anathema to most GOP politicians. Many sign a pledge promising not to increase them. How can Mr. Trump lead the Republican presidential field while calling for higher taxes? A look at poll data suggests that a constituency exists for raising taxes on the wealthy—even within the Republican Party.