Later, at the evening rally in Wilkes-Barre, Mr. Trump raised more concerns about voting fraud. “I just hear such reports about Philadelphia,” he said. “I hear these horror shows, and we have to make sure that this election is not stolen from us and is not taken away from us.”

He added for emphasis: “Everybody knows what I’m talking about.”

The crowd chanted an anti-CNN epithet as Mr. Trump attacked the “crooked media.”

At a rally in West Palm Beach, Fla., on Thursday, Mr. Trump said that, “This election will determine whether we remain a free nation or only the illusion of democracy,” suggesting that the system was “in fact controlled by a small handful of global special interests, rigging the system.” He continued, “And our system is rigged.”

The country has not had a presidential candidate from one of the two major parties try to cast doubt on the entire democratic process and system of government since the brink of the Civil War, said Douglas Brinkley, a presidential historian at Rice University.

“I haven’t seen it since 1860, this threat of delegitimizing the federal government, and Trump is trying to say our entire government is corrupt and the whole system is rigged,” Mr. Brinkley said. “And that’s a secessionist, revolutionary motif. That’s someone trying to topple the apple cart entirely.”

Roger J. Stone Jr., a close confidant and informal adviser to Mr. Trump, has also highlighted fears of election rigging. In an August column in The Hill, he wrote of voting machine manipulation. And during a panel Saturday at this year’s New Yorker Festival, as he discussed the possibility of such tampering, Mr. Stone hedged when asked whether he would advise Mr. Trump — should he lose in November — to concede the election and accept its legitimacy.