A reader writes, “Everyone says Donald Trump can’t win. Exactly why not?” David Leonhardt, editor for The Upshot, considers the question: Can Donald Trump win the Republican nomination? Sure, it’s within the realm of plausibility; he’s now been leading the polls for months, after all. But it seems highly unlikely because it would violate just about every historical pattern of presidential races. No modern candidate has received a major-party nomination — and perhaps no candidate in American history — while being opposed by the party’s elites: donors, media figures, politicians and others. Elite support matters because they have the ability...