"I will accept a clear election result, but I will also reserve my right to contest and file a legal challenge in the case of a questionable result."
In 240 years of quadrennial presidential elections in the United States, occasional very close outcomes have been disputed, because of perceived voting irregularities or mistaken vote counts.
In the third and final presidential debate with Democratic rival Hillary Clinton, Trump said he would wait to decide whether the outcome was legitimate.
They traded barbs on their Twitter accounts ahead of their high-stakes face-to-face encounter.
"I have never seen in my lifetime or in modern political history any presidential candidate trying to discredit the elections and the election process before votes have even taken place."
With three weeks until Election Day, the Clinton campaign says it will spend $2 million more in advertising in the Western state of Arizona, which has voted for a Democratic presidential contender only once in the last 16 elections.
A contestant in Trump's television show, "The Apprentice," told reporters Friday that the businessman kissed and grabbed her at a hotel.
"These claims are all fabricated. They're pure fiction and outright lies. These events never, ever happened and the people who said them fully understand."
"He was like an octopus," Jessica Leeds told the Times. "His hands were everywhere."
“He speaks his mind. He’s not afraid to say what is on his mind. He’s got gall.”
Zimbabwe's President Robert Mugabe: In 2007, authorities arrested opposition leader Morgan Tsvangirai at a rally and later beat him.