ST. LOUIS – Donald Trump mocked, disparaged, interrupted, and painted Hillary Clinton as a liar during their debate Sunday night.

But after 90 incendiary minutes, Trump gave his opponent a gift. He endorsed one of the central arguments of Clinton’s bid for the White House -- and her campaign is guaranteed to exploit it.

“She doesn’t quit. She doesn’t give up. She’s a fighter,” the Republican nominee said at the end of a night noteworthy for its venom rather than its grace. “I consider that to be a very good trait.”

Trump’s otherwise aggressive approach (the two rivals declined to shake hands at the outset) melded the flamboyance of his rallies and the stagecraft he learned as a reality TV personality. He walked into the hall determined to halt his campaign’s downward spiral following the release Friday of a damaging audio recording in which the businessman bantered in crude terms about using his fame as an opening to kiss and grope women.

The repudiations that followed the broadcast of the 11-year-old “Access Hollywood” clip had placed the trajectory of the Republican nominee’s campaign in serious doubt. Trump used the debate to appeal anew to the Republican supporters who oppose Clinton but were looking to Trump for assurances that the conservative outsider will continue for the next 30 days to challenge the former secretary of state on everything from Syria policy to her personal emails: “She never will change,” he argued.

The level of animus and name-calling between the rivals was at times cringe-inducing. The former first lady repeated her charge that Trump is unfit to be president and continues to be a purveyor of racist and religiously intolerant ideas.

But if Clinton hoped to use the debate to mitigate her trust deficit with voters, erase doubts about her use of a private email server, or defend her Libya policies or her lucrative speeches to Goldman Sachs, it was not clear she succeeded. She found herself playing defense, pleading with viewers to go to her campaign website to fact-check Trump.

During one exchange when Clinton said Americans should be relieved Trump is not in charge of enforcing the country’s laws, the real estate mogul shot back, “because you’d be in jail.”

The Washington University audience broke into applause following Trump’s reference to jail and his pledge to seek a special prosecutor to investigate Clinton if he’s elected. The clapping ignored admonishments for silence from moderators Martha Raddatz of ABC News and Anderson Cooper of CNN, who had their hands full trying to get the candidates to stop berating each other long enough to answer questions posed by undecided voters selected by the Gallup Organization.

In Missouri, Trump’s lead over Clinton in the RealClearPolitics polling average was 10 percentage points before the broadcast of the leaked video prompted House and Senate GOP lawmakers and prominent figures such as former Secretary of State Condoleezza Rice to denounce Trump as unfit to be president.

Trump repeated explanations he issued Friday that his “Access Hollywood” banter when he was 59 years old was “locker room talk” and not the boasts of a sexual harasser.

The Republican nominee, whose sturdiest support comes primarily from non-college-educated white men, cannot defeat Clinton in key swing states without broadening his appeal beyond that base. It was unclear whether his testosterone-infused debate performance helped narrow the double-digit gender gap.

Trump clearly had additional aims in mind, as well. He wanted to showcase more debate preparation, delve deeper into policy -- an area where Clinton continued to outpace him by miles -- and use his policy arguments to appeal to voters in states he needs to win. He mentioned West Virginia, Ohio and Pennsylvania, for instance, while answering a question about energy and climate change.

Trump’s policy weaponry was sharper while criticizing Clinton’s judgment and agenda than it was when detailing his prescriptions to create jobs, defeat ISIS or replace the Affordable Care Act enacted in 2010.

At one point, the GOP nominee even rebuffed his running mate, Indiana Gov. Mike Pence, over the ticket’s proposed approach to the civil war in Syria. “He and I haven’t spoken and I disagree,” Trump said brusquely when asked if he affirmed Pence’s position described last week during the vice presidential debate.

Clinton and Trump are scheduled to debate for the third and final time in Las Vegas on Oct. 19.

For weeks, Trump has hinted he was prepared to take aim at Bill Clinton’s reputation while serving as Arkansas governor and president, arguing his extramarital behavior was fair game because Hillary Clinton defended her husband by publicly denigrating and disputing some of his female accusers.

John Podesta, chairman of Clinton’s campaign and a former top White House adviser during many of the investigations of the Clintons in the 1990s, conceded after the debate that Trump sought to “throw her off her game.”

The St. Louis event was advertised as a town-hall format in which the questions would come from the audience and social media, but the release of the 2005 video over the weekend altered the terms of the debate, just as it did the dynamic of the campaign.

With prominent Republicans denouncing Trump, some 40 GOP candidates rescinding their support, and his own running mate saying he was “praying” for the nominee, moderator Cooper hit Trump hard with the second question of the night.

“We received a lot of questions about the tape released on Friday,” Cooper said. “You called what you said locker room banter -- kissing women without consent, grabbing their genitals. That is sexual assault. You bragged that you have sexually assaulted women.”

Shaking his head no, Trump responded, “I didn't say that at all. I don't think you understood what was said. This was locker room talk. I apologized to my family, to the American people. Certainly I’m not proud of it. … Yes, I'm very embarrassed by it. I hate it. But it's locker room talk.”

When Cooper kept pressing him, Trump did exactly what he signaled he’d do earlier in the evening: He went after his opponent and her husband on the same grounds.

“If you look at Bill Clinton -- far worse,” Trump said. “Mine are words, his was action. This is what he has done to women. There’s never been anybody in the history of politics in this nation that's been so abusive to women, so you can say any way you want to say it, but Bill Clinton was abusive to women.”

“Hillary Clinton attacked those same women and attacked them viciously,” Trump added. “Four of them are here tonight.”

And indeed they were. The women Trump was referring to are Kathleen Willey, Juanita Broaddrick, Kathy Shelton, and Paula Corbin Jones. Although all their cases are different, all four feel ill-used by the Clintons.

Willey was living in Richmond, Va., with her husband in 1993 and was acquainted with the Clintons because she had worked on the Democrats’ 1992 presidential campaign. After her husband suffered financial setbacks Willey inquired about a job in the administration.

She asserted on CBS News “60 Minutes” in 1998 that when she met with Bill Clinton in the Oval Office five years earlier expecting to talk about possible employment, the president hugged her and fondled her and placed her hand on his crotch. In an interview with the special counsel’s office during the investigation that would lead to his impeachment, Clinton denied doing so.

Broaddrick, now 73, said in 1999 that Bill Clinton raped her in a Little Rock, Ark., hotel room while he was running for governor in 1978. Clinton initially refused to comment on her allegations, but David Kendall, his private attorney, eventually denied on his client’s behalf that the encounter took place. To many Clinton critics, and even some supporters, Broaddrick’s allegation is the most troubling because three of her friends are on record as saying that she described the episode to them contemporaneously.

Shelton, now 54, was sexually assaulted when she was 12 years old by a 41-year-old factory worker, who was charged criminally. Hillary Clinton, then a Little Rock corporate attorney, was chosen as the court-appointed lawyer for the accused. As required by the ethics of her profession, Clinton mounted a vigorous defense, including challenging the credibility of the youthful victim.

In the end, the charge was reduced from forcible rape to unlawful touching of a minor and the defendant served less than a year in jail. The main controversy over this case -- and Trump alluded to it Sunday night -- is not the light sentence, but Hillary Clinton laughing about the case later. The context for this claim is an interview Mrs. Clinton later had with Arkansas journalist Roy Reed, who was writing about her legal career. Hillary brought up the rape case.

“Of course he claimed he didn’t [do it],” she recalled. “He took a lie detector test. I had him take a polygraph, which he passed, which forever destroyed my faith in polygraphs.” Clinton, speaking with a Southern twang, laughs as she makes this last statement.

Paula Jones, known as Paula Corbin at the time, is the woman who started it all. She sued Bill Clinton while he was president, asserting that he’d sexually harassed her while he was governor of Arkansas and she was a state employee. Specifically, she said that while working at a state conference, Clinton pulled her into an empty room, exposed himself to her and asked for oral sex. It was this case that led indirectly to the Monica Lewinsky scandal.

“So don’t tell me about words,” Trump said Sunday night. “Absolutely, I apologize for those words … but what President Clinton did, he was impeached, lost his license to practice law. He had to pay an $850,000 fine to one of the women, Paula Jones, who's also here tonight. And I will tell you that when Hillary brings up a point like that and talks about words that I said 11 years ago, I think it's disgraceful and I think she should be ashamed of herself.”

In response, Clinton replied, “First, let me say so much of what he just said is not right, but he gets to run his campaign any way he chooses. He gets to decide what he gets to talk about.”

“Instead of answering people’s questions, laying out the plans we have that make a better life and a better country,” she continued. “That's his choice. When I hear something like that, I am reminded of what my friend Michelle Obama advised us all: ‘When they go low, you go high.’”

Clinton did not defend her husband (Bill Clinton was seated in the hall next to daughter Chelsea) or even mention his name in this context. But she was not surprised by the line of counterattack because Trump had appeared before the cameras with the four women before the debate and posted it on Facebook.

After the debate, all four went to the room reserved for the hordes of media. Shelton, seeing the cameras, appeared overwhelmed, and left. But Willey, Broaddrick, and Jones -- as they have done in the past -- explained why they believe the Clintons should be held accountable, and why they are backing Trump.

"Mr. Trump may have said some bad words,” Broaddrick said. “But Bill Clinton raped me and Hillary Clinton threatened me. I don’t think there’s any comparison.”

When a journalist said to Trump as he was flanked by the four, “Do you touch women without their consent?” Paula Jones ended the session by snapping at him, “Why don’t you go ask Bill Clinton that?”

Tom Bevan contributed to this story.

Alexis Simendinger covers the White House for RealClearPolitics. She can be reached at asimendinger@realclearpolitics.com. Follow her on Twitter @ASimendinger.