In short, the challenge for Monday’s audience is to avoid the trap of thinking of this debate as yet another opportunity for “the real Trump” — or even a “new Trump” — to emerge, either stylistically or substantively. It’s way too late for that. The real Trump has been before the citizenry ever since he announced his candidacy in a rambling jeremiad that blamed Mexico for “sending” “rapists” to the United States as illegal immigrants. It has been said that the true test of an ordinary person’s character is how you behave when no one is watching. The corollary standard for a presidential candidate could be: how you behave repeatedly in public, before the one big night when everyone is watching. Even by that more forgiving standard, Mr. Trump has already flunked.