In an earlier exchange, when Clinton said it was unfortunate that Trump paints a dire picture of the livelihoods and economic circumstances of many African-Americans, Trump groaned in apparent disgust.
The 95-minute debate at Hofstra University on New York's Long Island pitted two historically unpopular and polarizing nominees against each other. The television networks were preparing for as many as 100 million people to watch, which would put Monday night's debate in the pantheon of the Super Bowl.
The clash came at a critical juncture in the campaign. With six weeks until Election Day, and with voters in some states already starting to cast ballots, polls show Clinton's summer lead has all but evaporated. Trump is effectively tied in many of the battleground states where Clinton had enjoyed comfortable leads.
Clinton poured forth with policy details and practiced catch phrases - "Trumped-up trickle down" to describe his tax plan, for instance - and tried to sow doubts about the seriousness of Trump's proposals. She seized on his comments about Russian President Vladimir Putin to suggest that Trump does not understand the global threats the country faces.
Where Clinton was measured in her attacks, Trump was a feisty and sometimes undisciplined aggressor. He regularly interrupted Clinton, as well as the moderator, "NBC Nightly News" anchor Lester Holt, and raised his voice. At times, Trump delivered rambling, heated and defensive answers.
Despite evidence to the contrary, Trump vehemently denied he had supported the Iraq War at the outset, as Clinton had, while Clinton looked on incredulously. Trump sought to blame Clinton for the growth of the Islamic State in Iraq and Syria, snapping, "You were secretary of state when it was a little infant."
Clinton mocked Trump's discussion of national security, suggesting he is uninformed and even unstable. "Whoo," she said with a laugh, when Trump finished one oration about NATO and the Islamic State.
Earlier, Trump grew visibly frustrated by Clinton's critique of his economic plan and declared: "Typical politician. All talk. No action. Sounds good. Doesn't work. Never gonna happen. Our country is suffering because people like Secretary Clinton have made such bad decisions in terms of our jobs, in terms of what's going on."
Trump, whose pugilistic aggression made him a dominant force in the Republican primary debates, began the first general-election debate with an uncharacteristically respectful tone. He ditched his campaign trail nickname of "Crooked Hillary" to call his opponent "Secretary Clinton."
"Is that OK?" he asked her. Clinton smiled. "Good," Trump continued. "I want you to be very happy. It's very important to me."
But Trump's demeanor quickly grew more aggressive, even bitter. He tried to portray Clinton as a relic of Washington and protector of the status quo. In one of his few dominant moments, he challenged Clinton on trade policy, saying the North American Free Trade Agreement and other trade pacts have contributed to the hollowing-out of America's middle class.
"Your husband signed NAFTA, which was one of the worst things that ever happened to the manufacturing industry," Trump said to Clinton. "You go to New England, you go to Ohio, you go to Pennsylvania - you go anywhere you want Secretary Clinton and you will see devastation."
Trump added: "You've been doing this for 30 years. Why are you just thinking about these solutions right now?"
Near the end of the debate, Trump repeated his claim that Clinton lacks what he sees as "the presidential look."
"She doesn't have the look. ... She doesn't have the stamina," Trump said.
Clinton looked with a smile, laughing.
"As soon as he travels to 112 countries," Clinton said, "he can talk to me about stamina."
That line drew loud applause in the hall.
Clinton continued. She said that Trump had tried to change the conversation from her "look" to whether she had stamina.
"This is a man who has called women pigs, slobs and dogs," Clinton said. "One of the worst things he said was about a woman in a beauty contest. He loves beauty contests, supporting them and hanging around them. He called this woman 'Miss Piggy,' and then he called her 'Miss Housekeeping,' because she is Latina," Clinton said. "She has a name, Donald."
Trump countered by suggesting that he had considered delving into the Clinton family's tawdry past on the debate stage. Over the weekend Trump had threatened to invite Gennifer Flowers, one of Bill Clinton's former mistresses, to attend the debate.
"I was going to say something extremely tough to Hillary, to her family, and I said, 'I just can't do it,' " Trump said.
Clinton accused Trump of postponing the release of his tax returns - something every presidential nominee has done for decades - because he has something to hide. Trump has said he is keeping his returns private at the advice of his lawyers because he is under federal audit.
Clinton speculated that Trump was "hiding" his tax returns because they would show he is not as rich as he says he is, or is not as charitable as he claims, or has debts to major banks and foreign entities, or pays nothing in taxes at all.
At that last suggestion, Trump scoffed, "That makes me smart."
Trump countered by offering to release his taxes if Clinton agreed to release her missing 33,000 emails. "I think it's disgraceful," Trump said of her use of a private email server as secretary of state. "And believe me, this country really thinks it's disgraceful also."
Clinton said, "I made a mistake using a private email."
"That's for sure," Trump interjected.
"I don't make any excuses," she continued.
With her concise answer, Clinton avoided the lawyerly details that have usually accompanied her discussion of the email issue, which her campaign staff has warned her sounds to voters like she is splitting hairs.
From the beginning, Clinton's strategy seemed in part to be to goad Trump to respond intemperately. Early on, she reminded the audience that "Donald was very fortunate," to the tune of what she said was a $14 million loan from Trump's father. Her father was a small businessman, Clinton added.
Trump, who is famously sensitive to suggestions that he owes his success to anyone else, took the bait. He used part of his next chance to speak to say he had received only a "small loan."
The exchange may seem petty, but it invokes central themes of the election, including the economic health of the middle class and which candidate is on the side of the little guy.
Clinton continued to press that case, charging that Trump took advantage of his workers and contractors who helped build his real estate assets.
"I have met a lot of the people who were stiffed by you and your businesses, Donald," she said. "I've met dishwashers, painters, architects, glass installers, marble installers, drapery installers, like my dad was, who you refused to pay when they finished the work that you asked them to do."
Clinton cited an architect who designed the clubhouse at one of his golf courses yet was not paid all he was owed. Trump retorted: "Maybe he didn't do a good job and I was unsatisfied with his work."
Trump went on to laud the achievements of his company, repeatedly saying his success reflects the kind of the thinking the nation needed in its political leaders.
In another exchange, Trump seemed rattled as Clinton accused him of saying that climate change "is a hoax, perpetrated by the Chinese."
"I do not say that, I do not say that," Trump interjected, shaking his head.
This was the first of three debates between Clinton and Trump sponsored by the nonpartisan Commission on Presidential Debates; the other two are Oct. 9 in St. Louis and Oct. 19 in Las Vegas. The vice-presidential nominees, Democrat Tim Kaine and Republican Mike Pence, will face off once, on Oct. 4 in Farmville, Virginia.
The third-party candidates did not qualify to participate in the debate because they did not meet a minimum polling threshold. Libertarian Party nominee Gary Johnson, who is positioned to be a potential spoiler in many states, and Green Party nominee Jill Stein both made appearances on campus Monday for media interviews. Stein staged a protest and at one point was ushered off campus by security because she did not have necessary credentials.
