"I will tell you at the time," he told moderator Chris Wallace. For days he has claimed the election is "rigged".

The Las Vegas debate continued the campaign`s bitter tone, with Mr Trump calling Mrs Clinton a "nasty woman".

Polls show Mr Trump is losing in key battleground states after facing a slew of sexual assault allegations.

The final battle of wits came less than three weeks before election day on 8 November.

The candidates declined to shake hands before and after the political sparring, setting the tone for what would later become yet another debate marked by shouting and interrupting.

Mr Trump appealed to the Republican establishment by vowing to appoint Supreme Court justices with a "conservative bent" who would overturn a key ruling that made abortion legal in the US and protect gun rights.

He also stuck to his pledge to deport undocumented immigrants and secure US borders.

Meanwhile, Mrs Clinton firmly declared she would stand up for the LGBT community, defend abortion rights, focus on restoring the middle class and equal pay for women.

"The government has no business in the decisions that women make," she said.

Other highlights from the debate at the University of Nevada, include:

- Mrs Clinton says Mr Putin wants Mr Trump elected because he wants a puppet as US president

- "You`re the puppet!" Mr Trump fired back, adding that Mr Putin had "outsmarted" and "outplayed" the former secretary of state

- Mrs Clinton said Mr Trump "choked" when he met with Mexican President Enrique Pena Nieto about his plan to build the wall, but he insisted it would happen under his presidency

- "We have some bad hombres and we are going to get them out," he said

- Mr Trump suggested Mrs Clinton and President Barack Obama coordinated the violence at his rally in Chicago earlier this year

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 Trump was.

While she was in the Situation Room helping to take out Osama Bin Laden, "he was hosting the Celebrity Apprentice", she said.

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