The Republican nominee came to Washington University awash in a scandal of his own making — the unveiling of 2005 hot-mic tapes of him denigrating women in lewd, aggressive tones.
After news hit of his "grab them by the pussy" chatter with TV host Billy Bush, Trump gave himself a hell of a balancing act to perform: Show voters sincere regret, while maintaining his signature swagger and deflecting Hillary Clinton's attacks.
In the end, the GOP standard-bearer relapsed to the strategy that won him the Republican primary — but may not be good enough to win him the presidency.
In short: He "let Trump be Trump."
Thomas Basile, a Republican commentator who analyzes politics for Forbes and Sirius XM radio, said Trump came into the arena with a lot on his plate: He both had to "look and sound prepared to be president" and understand "that people already think Hillary Clinton is 'crooked,' that the Clintons have questionable ethics and that people view her as untrustworthy."
Even so, "sticking to those points won't broaden his appeal or reassure Republicans," Basile warned. "As the untested candidate, Trump has to give the undecideds a reason to vote for him, not just against Clinton."
Clinton, meanwhile, just "had to [show] she can be president and let Donald Trump continue being Donald Trump," Democratic strategist Mike Morey said.
The St. Louis showdown was adversarial from the very start — the candidates skipped the traditional handshake.
It didn't get much warmer from there.
Minutes in, Trump had to answer for his objectification of women as captured on the Access Hollywood tapes, revealed Friday by the Washington Post.
Trump had already addressed his alpha-male screed in an unusual straight-to-camera apology. Sunday, he seemed anxious to briefly acknowledge his regrets and then sharply pivot to talking about Clinton's issues rather than rehash his own mistakes.
The ex-reality show star had already set the stage for 90 minutes of pushback. Just prior to the debate, he appeared alongside three women who have accused former President Bill Clinton of sexual assault.
That play was the opening act of one of the most stunning debates in American presidential history. If it was meant to rattle Clinton, noted for her studious and deliberate debate prep if not for flashiness, it's not clear that it did.
Nor is it a given the evening moves the needle for Trump, who's alienated members of his own party with his incendiary stances and, more recently, his obscene mouthings. Survival is one thing, but progress is another.
Trump's headlong rush to dismiss his graphic prattle as mere "locker room talk" and go after Clinton might have made sense from both a personal and tactical standpoint.
But that tactic both lacked humility and created a rhetorical roller coaster effect — Trump jumped from defending himself, to talk of defeating ISIS, to accusations against Clinton's husband and Clinton's email issues.
Clinton had her "I told you so" moment about her opinion of Trump's fitness for the Oval Office. She quoted first lady Michelle Obama, who has said of Trump and the GOP, "When they go low, we go high."
Trump did score with the audience during a tumultuous exchange on the emails: When Clinton witheringly expressed thanks that "someone with the temperament of Donald Trump is not in charge of the law in our country," Trump snapped back, "you'd be in jail."
No surprise either that Trump, who's actively demonized the "crooked" mainstream media to the glee of his fans, peevishly went off on the moderators. He scoffed that they'd teamed up with Clinton to silence him, "one on three."
Such cuts of red meat are guaranteed to sate people who already want Trump to beat Clinton.
The rub: With less than a month away from Election Day, Trump's challenge is to not only motivate his existing base to get out and vote, but to draw independent and undecided voters to his cause.
Morey, the Democratic strategist, unsurprisingly thought Trump missed the mark.
"Whether you are a dyed-in-the-wool Democrat, an undecided voter or a Republican, if you watched the debate, [you walked] away knowing Hillary Clinton is up to the job and can be president, regardless of whether you agree with her politically or not," he said.
Trump, who's changed campaign advisers about as often as he's retooled some of his stances, had a sparkling chance to reach out when a black man in the audience, James Carter, asked the candidates, "Do you believe you can be a devoted president to all the people in the United States?
The Republican, panned time and again as tone-deaf (at best) on race issues, began. "Absolutely. She calls our people 'deplorable,' a large group, and irredeemable," Trump said, referring to a Clinton dis that generated so much backlash she later apologized.
But in classic Trump style, he quickly veered away from a positive vow to be "a president for all of the people" to busting on Clinton as harboring "tremendous hate in her heart." He also returned to worn assertions about minorities being trapped in inner-city poverty.
Overall, Basile said after the debate, "If you were a wavering Republican, it is possible there was enough from Trump, particularly on his tax policy, Obamacare fixes and Supreme Court statements to prevent you from jumping ship."
But Basile also noted that Trump "may not have expanded the base, which is the name of the game at this point."
The bottom line is that, like his entire improbable campaign, Trump's Sunday performance reflected his style: brash, unapologetic, sometimes touchy and often prone to exaggeration.
Strategists and fretful GOP leaders alike have learned you can lead Trump to a teleprompter, but you can't always make him read it. He seems to have an irresistible urge to rewrite the script.
The final presidential debate is Oct. 19 in Las Vegas. Those who hope to see a radically different Donald Trump there are playing roughly the same odds as those who imagined he'd beg the nation's forgiveness in St. Louis.
Win or lose, even in a fight for the presidency, that just ain't who he is.