But the general’s real motivation here is to tell the public that Mr. Trump lacks the character to be President and should be defeated in November. “Donald Trump is the first president in my lifetime who does not try to unite the American people—does not even pretend to try. Instead he tries to divide us,” Mr. Mattis said. “We are witnessing the consequences of three years of this deliberate effort. We are witnessing the consequences of three years without mature leadership.”

This will resonate with many voters because it comes from someone who is no left-winger and has worked closely with the President. The question is whether this will begin a cascade of similar declarations from other former advisers. John Bolton, a former national security adviser, has written a memoir that will not be admiring…

Every President has breakups with advisers, but Mr. Trump has gone through them like an assembly line. His demand for personal loyalty and his thin skin clash with people who care about larger causes and have strong views. Mr. Trump’s habit of blaming others for policy decisions or events that go wrong also builds resentment. This was bound to boomerang as he ran for re-election, and so it is.