The most interesting thing Donald Trump has said recently isn’t his taunting of Hillary Clinton, it’s his comment to Bloomberg’s Joshua Green. Mr. Green writes: “Many politicians, Trump told me, had privately confessed to being amazed that his policies, and his lacerating criticism of party leaders, had proved such potent electoral medicine.” Mr. Trump seemed to “intuit,” Mr. Green writes, that standard Republican dogma on entitlements and immigration no longer holds sway with large swaths of the party electorate. Mr. Trump says he sees his supporters as part of “a movement.”

What,...