Trump's campaign was in free fall when he entered the debate hall Sunday night, reeling from the revelation of a 2005 video in which he spoke of women in lewd and sexually aggressive terms. The video sparked a dramatic rebuke of Trump, with dozens of Republicans in Washington and around the country saying the billionaire should step aside and let his running mate, Indiana Gov. Mike Pence, lead the GOP ticket.
The icy tone was set early when decades of tradition eroded as Trump and Clinton declined to shake each other's hand.
Trump fought back in the only way he knew how -- throwing out a battery of vicious counter punches. He vowed to prosecute Clinton if he is elected, and then throw her in jail. With her husband and daughter in the audience, Trump branded Bill Clinton a serial abuser of women hours after appearing alongside several women who allege the former president assaulted them.
Trump seemingly dismissed the significance of the vulgar language he was caught on tape using toward women a decade ago, apologizing for his conduct but repeatedly saying his remarks were just "locker room talk" that did not reflect his real character.
Clinton, however, said that incident did provide a true picture of Trump's character. She spoke of her years fighting Republicans on policy but said she never questioned their fitness to serve as president until now.
"Donald Trump is different," Clinton warned.
In the process, it seemed as if American politics changed in the course of one nasty night. The once sacred tradition of a presidential debate -- where candidates typically trade barbs over their vision of the country's future instead of personal humiliations -- exploded into something quite chilling.
Trump glowered, interrupted, and prowled the stage at Washington University in St. Louis, calling Clinton a "devil" and "liar" with "hate in her heart." The performance likely electrified his fiercely loyal supporters but may have done little to widen his appeal among more moderate swing state voters.
A CNN/ORC poll found 57% of debate watchers thought Clinton won compared to 34% who thought Trump came out on top. The poll only represents the views of people who watched the debate and has a slight Democratic advantage compared to CNN polls of all Americans.
Clinton didn't take Trump's bait, staying calm when he declared "Bill Clinton was abusive to women. Hillary Clinton attacks those same women." She didn't take the bait, repeating first lady Michelle Obama's philosophy articulated at the Democratic convention: "When they go low, we go high."
Pacing the stage
Trump paced the stage for much of the debate, which was moderated by CNN's Anderson Cooper and ABC's Martha Raddatz. He frequently interrupted Clinton and had trouble standing still while she spoke, sometimes appearing in her camera shot. He lost his composure at one point after a fierce exchange with Clinton about her emails, accusing the moderators of not addressing the issue even though Raddatz had asked a question about it.
"Nice, one on three" Trump said, claiming that the moderators were biased against him.
Hours earlier, Trump made a surprise appearance with women who have in the past accused Bill Clinton of inappropriate sexual advances. The women later attended the debate as members of the audience.
During the debate, Trump came across as more serious, prepared and less prone to losing his cool than in the first presidential debate two weeks ago when he was seen as the clear loser.
Clinton was less dominant than in the first debate and sometimes less effective in parrying Trump's attacks in an encounter far more frosty and bitter than in their first clash. But no other presidential candidate in history has faced the personal buzzsaw that Trump represented on Sunday night.
And Trump did not just aim his fire at the Clintons.
He took a swipe at Pence, who declined to go out in public and defend his running mate after the emergence of the video on Friday.
Trump slapped down Pence's statement during last week's vice presidential debate, when the Indiana governor said the US should be prepared militarily to target Syrian government assets to help alleviate the siege of the key city of Aleppo.
"He and I haven't spoken, and I disagree," Trump said.
After the debate finished, Pence tweeted he was "proud to stand" with Trump and congratulated him on a "big debate win." And Kellyanne Conway told CNN's Dana Bash that Pence and Trump spoke after the debate.
Syria was one of the few foreign policy items tackled during the debate. While Trump was far more polished in attacking Clinton's liabilities such as her email server and the weakest aspects of her record as secretary of state. But he often struggled when the debate turned to in-depth policy questions.
He suggested he would align himself with some of America's most hostile enemies, including Russia and the government of Syrian President Bashar al-Assad, to fight ISIS.
"I don't like Assad at all, but Assad is killing ISIS," Trump said. "Russia is killing ISIS. And Iran is killing ISIS. And those three have now lined up because of our weak foreign policy."
Obama administration officials have repeatedly said Russia and Syria are not focusing on ISIS but are instead targeting the moderate opposition, some of which are backed by the US, and defenseless Syrian civilians.
Clinton, meanwhile, branded her rival as outside the American mainstream and unfit to be president.
"He never apologizes for anything to anyone," Clinton said, hammering Trump for his treatment of the parents of a Muslim solider killed in Iraq, his disparagement of a judge with Mexican heritage, for his mocking of a disabled New York Times reporter and his birtherism crusade against President Barack Obama.
Perhaps the most stunning moment of the debate occurred when Trump threatened to use the power of the American government to investigate Clinton over the private email server she used as secretary of state.
"If I win, I am going to instruct my attorney general to get a special prosecutor to look into your situation. There has never been so many lies, so much deception. There has never been anything like it," Trump said.
Clinton replied: "it's just awfully good that someone with the temperament of Donald Trump is not in charge of the law in our country."
Trump hit back: "Because you'd be in jail."
The exchange was remarkable, even in an election campaign that has repeatedly redefined the scope of acceptable political discourse. Eric Holder, President Barack Obama's former attorney general, responded on Twitter: "So @realDonaldTrump will ORDER his AG to take certain actions. When Nixon tried that his AG courageously resigned. Trump is dangerous/unfit."
Asked by one of the undecided voters in the town hall style debate audience to find something she admired about Trump, Clinton mentioned his family, who she said had inherited their father's drive.
"His children are incredibly able and devoted, and I think that says a lot about Donald," Clinton said of his family, who attended the debate as did her husband, daughter and son-in-law.
Trump admitted to some grudging respect for Clinton's resilience.
"I will say this about Hillary: She doesn't quit. She doesn't give up. I respect that. I tell it like it is. She's a fighter," Trump said.
The night ended with a tepid handshake that was missing at the outset before the rivals retreated to the embrace of their families to lick their wounds.