With just 20 days before Americans cast ballots November 8, polls showed Clinton widening her lead over the provocative billionaire; AFP reported.
The 90-minute debate at the University of Las Vegas, Nevada offers Trump what may be his last chance to roll the political dice and reverse a campaign that has been battered by a stream of allegations he groped and forced himself on women.
"This is our final shot, folks," Trump told supporters Tuesday in Colorado.
Clinton has kept a low profile in recent days, shutting herself in with aides to prepare for the third televised debate of the campaign.
Moderated by Fox News journalist Chris Wallace, the event begins at 0100 GMT, before a televised audience expected to top 50 million.
The candidates will be asked about the economy, immigration, the Supreme Court, foreign flashpoints and their fitness to be president.
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