Suppose Mr. Trump keeps a cool head, conducts a respectful discussion with Ms. Clinton and Mr. Holt and even manages to avoid saying anything inflammatory or blatantly false. In other words, suppose he manages to conduct himself “presidentially” for an hour and a half. That could not undo the many, many instances, over more than a year — longer if you start with the launch of his “birther” campaign in 2011 — in which he has insulted, acted out, lied and countenanced violence beyond even some of the most rough-and-tumble precedents of modern American politics. Suppose, further, that he were to soften or even repudiate some of his most odious policy pronouncements; that, say, he opposes rather than supports the aggressive torture of terrorism suspects. That would be a backhanded form of progress, to be sure. But voters would still be left guessing as to which of his inconsistent statements they could trust.

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.