Republican presidential nominee Donald Trump suggested Monday that he fears the general election "is going to be rigged" — an unprecedented assertion by a modern presidential candidate.

'That election is going to be rigged': Trump 0:34 Trump's extraordinary claim — one he did not back up with any immediate evidence — would, if it became more than just an offhand comment, seem to challenge the very essence of a fair democratic process.

"I'm afraid the election is going to be rigged, I have to be honest," the Republican nominee told a town hall crowd in Columbus, Ohio. He added that he has been hearing "more and more" that the election may not be contested fairly, though he did not elaborate further.

Trump made the claim after first suggesting that the Democrats had fixed their primary system so Hillary Clinton could defeat Bernie Sanders.

Trump has previously backed up that thought by pointing to hacked emails from the national party that appeared to indicate a preference for Clinton. Still, the former secretary of state received 3.7 million more votes than Sanders nationwide and had established a clear lead in delegates by March 1.

The celebrity businessman — who has been known to dabble in conspiracy theories, including claims that President Barack Obama was not born in the United States and, more recently, that Senator Ted Cruz's father was an associate of former president John F. Kennedy's assassin — also claimed that the Republican nomination would have been stolen from him had he not won by significant margins.

He then asserted that November's general election may not be on the up-and-up.

'It's going to be taken away from us'

He repeated the charge Monday night on Fox News Channel's Hannity, saying: "November 8th, we'd better be careful, because that election is going to be rigged. And I hope the Republicans are watching closely or it's going to be taken away from us."

Requests to Trump's campaign for additional explanation were not returned.

Trump has not been shy about asserting that the electoral process has been "rigged."

It became a frequent catchphrase of his during a low-water mark of his primary campaign this spring, when forces allied with Republican rival Cruz managed to pack state delegations with supporters of the Texas senator.

Trump also asserted that the Republican Party had changed the delegate allocation in the Florida primary to favour a native candidate, like Jeb Bush or Marco Rubio, at Trump's expense.

In recent weeks, in an effort to woo angry Sanders supporters to his campaign, Trump has made the claim that the Democrats' process was also rigged.

Monday night, Trump said Sanders "made a deal with the devil," and said of Clinton, "She's the devil."