Few professional analysts seriously expected six months ago that Donald Trump could win the Republican nomination. But after Tuesday night, it is a clear possibility. How much of a possibility? To get that answer right, it is important to understand some things about the Trump phenomenon. Perhaps most important, Trump's campaign is not a Tea Party phenomenon. While there's been a tendency to try to lump Trump in with various G.O.P. insurgencies of the last few years, that role is best filled by Sen. Ted Cruz, who really occupies the "Tea Party lane" of the G.O.P. primary more cleanly. Instead,...