Republican lawyers say it is impossible for the US presidential election to be "rigged" against Donald Trump, despite such claims by the Republican nominee.
Trump, a New York businessman making his first run for public office, has sought to raise fears of a flawed election as he has fallen in opinion polls against Democrat Hillary Clinton.
"Of course there is large scale voter fraud happening on and before election day. Why do Republican leaders deny what is going on? So naive!" Trump said on Twitter on Monday.
Numerous studies have shown voter fraud in US elections is very rare.
Republican campaign lawyer Chris Ashby said Trump's charges, which were not backed by any evidence, could foment unrest and were "unfounded" and "dangerous".
"When you say an election is rigged, you're telling voters, your supporters, their votes do not matter," Ashby said in an interview.
"I think some of Donald Trump's comments could cause unrest at the polls."
Some Republicans have urged Trump to drop the assertions.
Early voting and voting by mail have already begun in many states.
Mark Braden, partner at Baker and Hostetler and former chief counsel for the Republican National Committee, said any sort of election rigging at the national level "just is impossible", citing the various systems that would make such an endeavour complicated and unfeasible.
"Our system is principally a system based upon each side watching the other side," Braden said in an interview.
"Our system is dependent on local volunteer participation. Our system has worked very well because we have people who get involved in the process and perform these functions."
Trump's vote-rigging accusations come after his campaign was damaged by the release of a tape 10 days ago showing him lewdly bragging about kissing and touching women without their permission.
A series of women have come forward with allegations about his behaviour but he has denied those accusations.
The RealClearPolitics average of national opinion polls shows Clinton leading Trump by 6.9 percentage points, at 45.9 per cent to 39 per cent.
Trump's vice-presidential running mate, Mike Pence, and his campaign manager, Kellyanne Conway, have tried to portray the candidate's comments about vote rigging as being aimed at an unfair news media.
But Trump has also pointed to fraud "at many polling places".
The country's top elected Republican, House of Representatives Speaker Paul Ryan, has also tried to counter Trump's message about election fraud.
Spokeswoman AshLee Strong said Ryan "is fully confident the states will carry out this election with integrity".
Clinton campaign manager Robby Mook said Trump's assault on the voting system was an act of desperation.
"He knows he's losing and is trying to blame that on the system. This is what losers do," Mook told reporters.
In a report titled The Truth About Voter Fraud, the Brennan Center for Justice at New York University School of Law cited voter fraud incident rates between 0.00004 percent and 0.0009 percent.
