"I will tell you at the time," Trump told moderator Fox News' Chris Wallace, after Wallace asked whether Trump would follow the American tradition of accepting the results of presidential elections. "I will keep you in suspense."
Clinton interjected.
"Chris, let me respond to that," she said. "That's horrifying."
Trump also referenced what he said was a "rigged" election: He blamed the news media for "poisoning" minds against him, and the FBI for not prosecuting Clinton for her use of a private email server to handle government business while she was secretary of state.
On women
The topic of Trump's treatment of women also figured large.
He rejected the accounts of women who have accused him of groping them or kissing them without their consent.
"I didn't even apologise to my wife, who's sitting right here, because I didn't do anything," Trump said, when asked by Wallace about nine women who have come forward in recent days.
Clinton said that Trump's treatment of women reflected a broader flaw in his personality.
"Donald thinks belittling women makes him bigger. He goes after their dignity, their self-worth," Clinton said. "And I don't think there's a woman anywhere who doesn't know what that feels like."
On ties with Russia
And earlier in the debate, Clinton accused her opponent of being a "puppet" of Russian leader Vladimir Putin.
"It's pretty clear, he'd rather have a puppet as president of the United States," Clinton said, noting that Russian hackers had been blamed for releasing internal Democratic emails, apparently in an effort to help Trump's cause.
"You're the puppet," Trump replied.
He argued that Putin had outmanoeuvred Clinton - who was previously secretary of state - and President Barack Obama in nuclear treaties, and in Middle Eastern conflicts like the one in Syria.
"She doesn't like Putin because Putin has outsmarted her at every step of the way," Trump said.
A last opportunity for Trump
For Trump, the debate was perhaps his last opportunity to turn around a race that appears to be slipping away from him. His predatory comments about women and a flood of sexual assault accusations have deepened his unpopularity with women and limited his pathways to victory.
Clinton took the stage facing challenges of her own. While the electoral map currently leans in her favour, the Democrat is facing a new round of questions about her authenticity and trustworthiness, concerns that have trailed her throughout the campaign.
The Democratic vice-presidential nominee said running mate Clinton would be cool and collected in the face of Trump's likely insults and histrionics.
Meanwhile, Republican vice-presidential candidate Mike Pence was predicting the debate would revolve around "bigger things" facing the nation, rather than controversies surrounding Clinton.
Psychological tactics
Both presidential campaigns have been using their debate guests as a tool to try to get inside their rivals' heads.
Trump is bringing Barack Obama's half-brother, Malik Obama, a Trump supporter. Clinton is bringing frequent Trump critic and billionaire Mark Cuban and Hewlett Packard Enterprise CEO Meg Whitman.
No matter how persuasive Trump and Clinton are, it's already too late for either to win over millions of voters.
Advance voting by mail or in person is now underway in more than 30 states. At least 2.1 million voters have already cast ballots.
More than 45 million people are expected to vote before Election Day, November 8.
- Additional reporting AP, Washington Post
