Donald Trump has suggested that this year’s election could be “rigged”.
The Republican nominee 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 Donald Trump called Democratic rival Hillary Clinton “the devil”.
He has come under fire from across the political divide for remarks he made about the parents of a Muslim soldier killed in Iraq.
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.”
Donald Trump has made similar comments before in relation to the Democratic race, suggesting the party fixed its system to favor nominee Hillary Clinton over her challenger Bernie Sanders.
Earlier this year, Donald Trump also complained the Republican primary system was also “rigged” amid party efforts to stop his march to the candidacy.
Correspondents say the New York billionaire could be trying to capitalize on voters’ distrust of establishment institutions, or lay the groundwork should he lose the election.
At another rally in Pennsylvania, Donald Trump took the unprecedented step of directly calling Hillary Clinton “the devil”.
He attacked Bernie Sanders for capitulating in the Democratic race, saying he “made a deal with the devil. She’s the devil”.
Democrats and Republicans alike have condemned Donald Trump for his remarks about the parents of Captain Humayun Khan, who was killed by a car bomb in 2004 in Iraq, at the age of 27.
Former Republican presidential candidate John McCain became the latest senior figure in the party to criticize Donald Trump for his attacks.
John McCain, a veteran of the Vietnam War, said in a strongly worded statement that Donald Trump did not have “unfettered license to defame the best among us”.
Donald Trump had caused controversy by suggesting Humayun Khan’s mother, Ghazala Khan, had been prevented from speaking alongside her husband at the Democratic convention last week.
In another development, Warren Buffett challenged Donald Trump to release his tax returns.
Donald Trump has said that they cannot be made public until the financial authorities have completed an audit.
However, Warren Buffett said there were no rules against showing tax returns and allowing people to ask questions about them.