This imminent presidential debate could very well be the most decisive two hours of this strange 2016 campaign and determine who will win on Nov. 8.
A lot can happen in the seven weeks remaining, of course. But Donald Trump and Hillary Clinton, the two most disliked White House candidates in modern history, each have much to prove - and disprove - under the nationally televised questioning of NBC's Lester Holt with some 60-plus million watching.
First, Clinton better not cough once.
It's hard to imagine more dissimilar styles and personalities than these two senior citizens, one of whom could become the oldest ever to take office and the other the second oldest. As it was in 2008, this race was long seen as Clinton's to lose. Recent polls indicate she is again succeeding at that, as the 70-year-old billionaire pulls even, and in some states ahead, of the former first lady, former senator and former cabinet member, who turns 69 next month.
Unexpectedly, Trump's task has become easier than hers. He just needs to look reasonable and informed.
Here's what's at stake on Sept. 26: Trump must appear strong, firm and, above all, presidential. He has much to live down given his brash, braggadocio and sometimes uncouth behavior during the primaries when he nonetheless vanquished 16 far more accomplished, polished and experienced party competitors.
His recent controlled behavior under new campaign advisers has encouraged supporters. And that impromptu foray into foreign affairs, meeting the Mexican president, produced a photo worth a good chunk of Trumps' reputed fortune.
Clinton must live up to her reputation, now fanned by the Democrat who defeated her in 2008, as the best-prepared presidential candidate in decades. She must defend a long record as senator and secretary of state, mixed at best.
She has numerous scandals of her own making to confront, including the lethal Benghazi affair, her email mess with its many accompanying lies and her family's private foundation that did so well financially during her cabinet days.
As always with the Clintons, she has explanations for almost everything, recited in such arcane detail that listeners' minds often drift off, as she intends.
Like Ronald Reagan, the reality celebrity knows what works on camera. And what's more reality than a real presidential debate?
That alarming video of a wobbly, would-be president collapsing on that New York curb is now seared into the minds of millions. She can laugh it off and once more admit she made a mistake, as she did with her personal email server, by not admitting misjudgment sooner.
That's also Clintonian, admit only what you must and not before you must, an opacity that over 25 years explains her awful trust poll numbers.
Clinton must also appear energetic, alert, fully-recovered from her pneumonia -- and display no suspicious signs of any other malady. The slightest unexplained pause or wobble will sound instant alarm bells.
She hates traditional debate prep. It is grueling, with days of cramming details, history and policy nuances. She has to memorize scripted responses to anticipated attack scenarios with a stand-in opponent hurling pointed insults. She once stormed from such a session.
No doubt Clinton also plans some verbal bait trying to set Trump off.
As the longtime boss of a personal empire, Trump is unaccustomed to in-your-face challenges. He claims to shun formal debate preparation. But Newt Gingrich explained last week Trump prepares all the time here and there. In the middle of lunch he'll say, If she says this, what should I reply?
He must be careful, however, with the dismissive, even crude, condescension he's displayed toward female competitors like Carly Fiorina or moderators like Fox News' Megyn Kelly.
This is a screen test for POTUS. Rolling eyes or that mocking "gimme-a-break" look could be even more damaging than Al Gore's gratuitous sighs as George W. Bush spoke in 2000.
Like midterm exams, debates seem crucial at the time, and they could sway many early voters. But they're rarely decisive; ask Mitt Romney, who completely disassembled President Obama in their first 2012 set-to.
Much campaigning and two more debates await in the 43 remaining days after this confrontation.
Backstage at these events surprised me. The principals are the calmest people around. At that high altitude of politics, participants relish walking a high-wire with everyone watching. Family and staff want to die.
Two things, however, are certain: As in the GOP primaries, Trump's presence will launch TV ratings through the roof. And the Pepcid consumption on both sides will be yuugge.
McClatchy Washington Bureau