Trump also used the policy speech to say that after the election he would sue the nearly one dozen women who have come forward to accuse him of sexual assault.
New data from the Pew Research Center indicated that more than 4 million ballots had been cast by Saturday, and that before November 8 arrives, the advance voting total may well swell beyond 50 million.
"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.
"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.”