Trump said on Thursday he would accept the results of a "clear election" but would reserve his right to contest the outcome.
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"And of course, I will abide by all the rules and traditions of all the candidates who have come before me," he said before promising that "we are going to win so big."
Trump has been criticized over the course of the campaign for failing to release his tax returns, a tradition that has been followed by presidential candidates for more than 40 years.
He reiterated claims that the upcoming election will be rigged.
"Some people vote even if they're dead, which is hard to do," Trump said. "But it's easy if there's fraud involved."
There is no evidence of large-scale voter fraud, as the GOP presidential nominee claims. A statement to that effect earned a "pants on fire" rating from Politifact, meaning that it has no basis in fact.
Trump has been widely condemned for the claim. At a news conference Tuesday, President Barack Obama criticized the GOP nominee.
"There's no evidence that that has happened in the past or that there are instances in which that would happen this time. And so I would advise Mr. Trump to stop whining and go make his case to try to get votes," Obama said.