Mr Trump’s claims were part of a Monday morning blast of tweets that took on his party, the women who have accused him of sexual misconduct, the media and US vice president Joe Biden.
The defensive barrage, interrupting a relatively quiet day on the campaign trail, comes as Republicans are under pressure to rebuke Mr Trump’s claims that the presidential election is “rigged” in Democrat Hillary Clinton’s favour.
The push reflects growing worries that their nominee’s unsubstantiated rhetoric could erode public trust in elections and lead to damaging disputes if he loses.
Mr Trump’s running mate, Mike Pence, declared the ticket will “absolutely accept the results of the election”.
But Mr Trump seemed to brush back against his vice presidential pick.
“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!” he tweeted.
There is no evidence to back up Mr Trump’s claim of widespread voter fraud.
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A study by a Loyola Law School professor found that out of one billion votes cast in all American elections between 2000 and 2014, there were only 31 known cases of impersonation fraud.
Mr Pence and other Republicans have tried to argue that Mr Trump is claiming that media bias is rigging the election in Democrats’ favour, particularly in the reporting of allegations by several women that Mr Trump had sexually assaulted them.
Mr Trump pushed back against those women on Monday, tweeting: “Can’t believe these totally phoney stories, 100% made up by women (many already proven false) and pushed big time by press, have impact!”
In another tweet, Mr Trump linked to a video montage of Mr Biden greeting and hugging women at various events.
The Democrat has not been accused of sexual improprieties, but has raised eyebrows for his lingering or awkward embraces of women.
The tweets abruptly returned the focus to the Republican candidate’s list of troubles on an otherwise quiet day on the campaign trail ahead of the final presidential debate on Wednesday.
Mr Trump is spending much of the day out of sight before campaigning in Green Bay, Wisconsin, on Monday evening.
Mrs Clinton is due to spend Monday preparing with advisers near her home in New York.
As the candidate prepares, Mrs Clinton’s campaign continues to have to answer for the contents of hacked emails being released by the thousands by WikiLeaks.
The most recent batch showed Mrs Clinton generally avoided direct criticism of Wall Street as she examined the causes and responses to the financial meltdown during a series of paid speeches to Goldman Sachs.
WikiLeaks said on Monday that founder Julian Assange’s internet access has been cut by an unidentified state actor. Few other details were immediately available.
The fraud concerns raised by Mr Trump are not new to Republicans.
They often cite the risk of fraud as they make a case for tightening access to the polls through voter identification laws and other restrictions.
But Mr Trump’s often repeated claim that the election is “rigged” has put the party in the unusual position of expressing faith in the legitimacy of the election system.
Leaders in both parties fear Mr Trump’s claims will encourage his most ardent supporters not to accept the results, leading to prolonged litigation, possible violence or hardened divisions - or some combination of the three.
That could make it difficult even to govern and take a long-term toll on the democracy.
Even staunch conservatives have found themselves in the position of trying to gently distance themselves from the nominee’s remarks.
“I just don’t think it’s that constructive to make this a campaign issue,” Representative Steve King said on CNN.
The staunch conservative from Iowa said he shares Mr Trump’s worries about election fraud, but acknowledged Mr Trump’s claims are “partially unsubstantiated”.
“I don’t want to say anything on this programme that de-legitimises our election,” Mr King said.