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Trump has been damaged recently by several accusations he groped women - which he denies - and concerns about his claims the election will be rigged against him.
The debate at the University of Las Vegas, Nevada, gives the New York businessman perhaps his best remaining chance to sway the dwindling number of Americans who are still undecided about which way to vote in the Nov. 8 election.
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| Republican U.S. presidential nominee Donald Trump listens as Democratic U.S. presidential nominee Hillary Clinton speaks during their third and final 2016 presidential campaign debate at UNLV. (Photo: Reuters) |
Tens of millions of voters are expected to tune in to the debate that starts at 9 p.m. EDT (0100 GMT October 20).
"This one is important if Trump is going to have any chance to get back into this race," said Republican strategist Charlie Black. "He's going to have to talk about issues effectively and not get down in the mud, and he needs to talk about jobs."
Trump campaign manager Kellyanne Conway acknowledged that her candidate goes into the debate as the underdog.
"It's a comeback and he's done it before several times in this campaign," Conway said on Fox News, adding a gibe at Clinton: "Hillary Clinton just has not been able to put him away. What is her problem already?"
Trump did not do a routine walk-through of the debate hall in preparation for the showdown.
For Clinton, who leads in national polls and in most of the battleground states where the election will likely be decided, the 90-minute debate is a chance to make a closing argument on why she is best suited to succeed fellow Democrat, President Barack Obama.
