DEMOCRAT HILLARY CLINTON and Republican Donald Trump will debate on national television for the first time Monday night, and the stakes could not be higher. The presidency and, by extension, the country’s future — maybe the world’s — could hinge on what they say and how they say it.

Or so we have been told — in breathless pre-event speculation about everything from whether the moderator, NBC’s Lester Holt, will intervene to correct a candidate who strays from the truth, to whether one candidate or the other will be able to goad his or her opponent into a campaign-altering gaffe before an audience expected to reach 100 million.

Permit us to dissent from this conventional wisdom, vigorously. Yes, Monday night’s clash, and two additional debates to follow, will add drama to the election, and a bit more data to the massive pile of it already available to voters. In a fundamental sense, however, there is nothing much at stake, or shouldn’t be, because there is not much more to learn: Mr. Trump has amply demonstrated his unworthiness to occupy the Oval Office. It’s beyond his capacity in the upcoming 90-minute question-and-answer sessions to reverse or even substantially modify that conclusion.

[Obama’s campaign guru: Don’t fret about polls. Clinton is winning, and she can finish the job tonight.]

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.

These six moments from the 2016 primary debates have the context you need before you see Hillary Clinton and Donald Trump go head-to-head in the first general election debate Sept. 26 (Sarah Parnass/The Washington Post)

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.