Hillary Clinton and Donald Trump traded insults and sparred over temperament, stamina and judgment Monday, in a fiery US presidential debate that often saw the Republican on the back foot.
With six weeks until election day and polls showing a virtual dead heat, the Democrat Clinton repeatedly questioned her rival's fitness to serve in the Oval Office.
Before an anticipated television audience of up to 100 million, Clinton painted the celebrity real estate mogul as fatally out of touch and willing to say "crazy things" to get elected.
"You live in your own reality" said the 68-year old Democrat, accusing Trump of launching his political career on the "racist lie" that Barack Obama is not American.
As Clinton projected steady experience, casting herself as the voice of reason, Trump played the populist bruiser, pitching to frustrated blue-collar voters fed up with politicians and wanting change.
"Let me tell you, Hillary has experience. But it's bad, bad experience," quipped the billionaire, accusing the former secretary of state, first lady and US senator of being a "typical politician. All talk, no action. Sounds good, doesn't work."
The 70-year-old Trump has faced tough questions about his temperament during a deeply polarizing 15 month campaign that has brought right wing populism to the American political mainstream.
He began the keenly awaited debate at New York's Hofstra University with a more restrained tone, even ditching his red power tie for a more statesmanlike blue.
But as the temperature rose, he quickly brought out the verbal brickbats, repeatedly interrupting Clinton and even questioning her stamina after a bout of pneumonia.
"Well, as soon as he travels to 112 countries and negotiates a peace deal, a ceasefire, a release of dissidents... or even spends 11 hours testifying in front of a congressional committee, he can talk to me about stamina," Clinton shot back.
If they were evenly matched at first, Trump appeared to get more irritated and riled, at one point rolling his eyes and emitting a frustrated "ugh."
In a snap CNN poll of 521 voters, 62 percent judged that Clinton had won the debate, against 27 percent for Trump.
If Clinton's brightest moments came when debate turned to foreign policy, Trump's came when he tapped into rich seams of malaise about politics and the economy.
"Our country is suffering because people like secretary Clinton have made such bad decisions in terms of our jobs," he charged.
Trump squarely blamed Clinton and the political class for losing jobs to Mexico and China through bad trade deals and incompetence, and snapped at her proposals to help a middle class still recovering from the Great Recession.
"You've been doing this for 30 years. Why are you just thinking about these solutions right now?"
Clinton tried to undercut Trump's CEO-in-chief acumen by accusing him of having "stiffed" small businesses throughout his career.
She demanded Trump keep with tradition and release his tax returns, suggesting he may be lying about his much-vaunted wealth, his charitable donations, his tax bill or his ties with foreign benefactors.
She also accused him of backing an economic platform amounting to "the most extreme" package of tax cuts for the wealthy in US history.
"I call it trumped up trickle-down, because that's exactly what it would be," she quipped.
Trump fired back that he would release his tax returns, "when she releases her 33,000 e-mails that have been deleted," alluding to the Democrat's use of a private email server as secretary of state.
Trump's biggest handicap may be accusations that he has a weak grasp of policy -- which he sought to counter by accusing his rival of sowing chaos in the Middle East during her tenure as secretary of state.
"It's a total mess, under your direction, to a large extent," Trump said.
But he appeared on shaky ground as he defended his refusal to reveal his plan for defeating the Islamic State group.
"You're telling the enemy everything you want to do. No wonder you've been fighting ISIS your entire adult life," he said of the group that only came to prominence in the last decade.
Clinton -- perhaps the most qualified presidential candidate since George Bush senior or Dwight Eisenhower -- has a massive organizational advantage, a bigger campaign warchest, a lead in the popular vote and is in a notably stronger position state-by-state.
But Trump weathered allegations of bigotry and sexism to triumph in a vicious Republican primary campaign, and now has a real shot at being sworn in as the 45th president of the United States on January 20.
This first 2016 presidential debate could be pivotal in deciding whether Clinton will become the first woman president, or if Trump can pull off the greatest upset in US political history.