One day of bad polls and Trump's feeling hopeless, telling a crowd that the November election could be "rigged" and adding that Hillary Clinton is the devil.
He told a rally in Columbus, Ohio, that he had heard "more and more" that the contest would be unfair. He offered no immediate evidence.
At another event he called Democratic rival Hillary Clinton "the devil".
Mr Trump has come under fire from across the political divide for remarks he made about the parents of a US Muslim soldier killed in action.
On the forthcoming vote, he told supporters "I'm afraid the election is going to be rigged, I have to be honest".
He later repeated the claim on Fox News, adding "I hope the Republicans are watching closely or it's going to be taken away from us."
To call another candidate "The Devil" is unprecedented in predominantly-Christian America, where locals often place deep stock in religious traditions.