US Republican presidential nominee Donald Trump has said the election is "absolutely rigged" by the "dishonest media" and "at many polling places".
His comments appear to contradict his running mate Mike Pence, who told NBC earlier Mr Trump would "absolutely" accept the election result, despite media "bias".
Mr Trump's adviser Rudy Giuliani has also accused Democrats of "cheating".
Polls suggest Mr Trump is losing ground in some of the key battleground states.
Meanwhile Hillary Clinton's running mate Tim Kaine has blasted Mr Trump's election-rigging claims as "scare tactics".
Mr Trump has questioned the legitimacy of the election process in a series of tweets, the latest of which said: "The election is absolutely being rigged by the dishonest and distorted media pushing Crooked Hillary - but also at many polling places - SAD".
Earlier, Mr Trump accused the press of inaccurate reporting: "Election is being rigged by the media, in a coordinated effort with the Clinton campaign, by putting stories that never happened into news!"
However, speaking on NBC's Meet The Press, Mr Pence said the American people were "tired of the obvious bias in the national media" which was "where the sense of a rigged election goes", but said: "We will absolutely accept the results of the election."
"Elections always get pretty rough," he added, but said the US has a tradition of "the peaceful transfer of power".
Meanwhile former New York Mayor Rudy Giuliani, who is Mr Trump's campaign adviser, told CNN's State of the Union that he'd have to be a "moron" to think that some elections, such as those in Philadelphia and Chicago, were going to be fair.
"I've found very few situations where Republicans cheat... they don't control the inner cities the way Democrats do. Maybe if Republicans controlled the inner cities, they'd do as much cheating as Democrats," he said.
"I'm sorry. Dead people generally vote for Democrats rather than Republicans," he added.