Republican candidate Donald Trump has refused to commit to accepting the US election result if he loses, in the final televised debate against his Democratic rival, Hillary Clinton.
"I will tell you at the time," he told the moderator, Chris Wallace from Fox News, at the debate at the University of Nevada in Las Vegas. For days, Mr Trump has claimed the election is "rigged".
The debate - the last before the presidential election next month - continued the campaign's bitter tone, with Mr Trump calling his opponent a "nasty woman".
It began with Mrs Clinton vowing to uphold women's and LGBT rights, while Mr Trump pledged to protect gun rights.
Polls show Mr Trump is losing in key battleground states after facing a slew of sexual assault allegations.
Most Americans will cast their votes on 8 November.
Highlights from the third and final debate included:
When asked about her paid speech to a Brazilian bank in which she spoke of her dream of open trade and open borders, Mrs Clinton said she was talking about energy policy.
In one key exchange, he attacked her 30 years of "very bad experience" and she responded by going through her timeline, comparing where she was to where Mr Trump was.
While she was in the Situation Room helping to take out Osama Bin Laden, "he was hosting the Celebrity Apprentice", she said.
Mr Trump has faced damaging fallout after a video emerged of him making obscene comments about groping women, with senior Republicans deserting him.
In recent days he has railed against the election process itself, warning the vote is "absolutely rigged" with "large-scale voter fraud" taking place.
His remarks prompted a scathing response from President Barack Obama, who said the Republican challenger should "stop whining".