"I'm afraid the election's going to be rigged. I have to be honest," Trump told voters in Ohio, a crucial swing state.
Trump's comments Monday came as he decried Vermont Sen. Bernie Sanders for endorsing Hillary Clinton in the Democratic primary, even as some Sanders supporters have continued to resist unifying behind the nominee. Trump has sought to siphon off Sanders supporters and draw them to his campaign.
Trump added that he has heard "more and more" that the November election will be rigged -- suggesting to his supporters that the outcome of the election is out of the hands of voters.
Hillary Clinton's spokesman Brian Fallon mocked Trump on Twitter Tuesday morning, writing, "Even for a reflexive conspiracy theorist like Trump, this is pathetic. It's dangerous, too."
Trump during the primary repeatedly slammed the "rigged system" he claimed was working against his campaign to capture the Republican nomination for president. He then pivoted to using that language to decry the nomination process on the left, accusing the Democratic Party of colluding with the Clinton campaign to keep Sanders from winning that party's nomination.
Trump's comments during the primary bolstered the impression that Trump, a political outsider, was leading the charge against a corrupt political system.
But his latest comments could hurt Trump's general election campaign as his supporters might decide not to turn out to vote if the election is already "rigged" against their candidate.
Trump continued with the "rigged" theme during a Monday night interview on Fox News. Appearing on "Hannity," the Republican nominee suggested the potential for foul play in November. Trump pointed to the 2012 presidential election as a cause for concern.
"I've been hearing about it for a long time," Trump said. "And I know last time, there were -- you had precincts where there was practically nobody voting for the Republican. And I think that's wrong. I think that was unfair, frankly" for 2012 GOP nominee Mitt Romney.
"I'm telling you, November 8, we'd better be careful, because that election is going to be rigged," Trump added. "And I hope the Republicans are watching closely or it's going to be taken away from us."