Every Trump critic had “Oh, hell” moments during the primary season. They were when Donald Trump demonstrated a keen, gut-level political instinct that even an exceptionally talented conventional politician would be hard-pressed to match. An example: During a Republican debate in Florida in February, Trump was asked about former Mexican President Vicente Fox’s comment that his country wouldn’t pay for Trump’s “[expletive deleted] wall.” “The wall just got 10 feet taller,” Trump shot back. The rejoinder was funny and memorable. A Republican senator told me that his cellphone instantly lit up with constituents thrilled at what Trump had said. In...