Republican presidential nominee Donald Trump has suggested that the November election could be "rigged".
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.
Mr Trump has made the claim before in relation to the Democratic race won by Hillary Clinton.
His comments come amid further criticism for remarks he made about the parents of a Muslim soldier.
Mr Trump repeated the rigging allegation later on Fox News.
"November 8th, we'd better be careful, because that election is going to be rigged," he said. "And I hope the Republicans are watching closely or it's going to be taken away from us.''
At another rally in Pennsylvania he made the unprecedented step of directly calling Democratic rival Hillary Clinton "the devil".
He attacked Bernie Sanders for capitulating to Mrs Clinton in the Democratic race, saying he "made a deal with the devil. She's the devil."
Democrats and Republicans alike have condemned Mr Trump for his remarks about a US Muslim soldier's parents.
Former Republican candidate John McCain became the latest senior Republican to criticize Mr Trump for his attack on the parents of US Army Capt Humayun Khan, who was killed by a car bomb in 2004 in Iraq, at the age of 27.
Senator McCain, a veteran of the Vietnam War, said in a strongly worded statement that Mr Trump did not have "unfettered license to defame the best among us".