Mr. Trump’s ominous claims of a stolen election which he often links to black, urban neighborhoods are not entirely new.
Donald J. Trump has lashed out at fellow Republicans, calling them disloyal and far more difficult than Hillary Clinton.
He has griped openly about a “rigged” political system, saying Wednesday that he has “no respect” for the nonpartisan Commission on Presidential Debates, and previously complaining about a “defective” microphone in the first debate.
And on Monday, at a rally in Wilkes-Barre, Pa., he worried that the election could be “stolen” from him and singled out Philadelphia, a city with a large African-American population, warning, “We have to make sure we’re protected.”
The assertions — which coincide with Mr. Trump’s decline in the polls after a shaky performance in the first debate and accusations that he forced himself on women — highlight concerns that he may not accept a Clinton victory, breaking from the traditional decorum of defeated presidential candidates and undermining the legitimacy of the election result.
At rallies in recent days, Mr. Trump has become a candidate full of excuses, perhaps the clearest manifestation of his frustration with his current standing in the polls and the growing alarm in his campaign that a White House victory is slipping away.
On Monday, on a trip through Pennsylvania, Mr. Trump began the day urging the almost entirely white crowd outside Pittsburgh to show up to vote, warning about “other communities” that could hijack his victory.
Early voting in Mesa, Ariz. Donald J. Trump is increasingly warning of voter fraud and railing against perceived news media bias. Credit Caitlin O’hara for The New York Times
“So important that you watch other communities, because we don’t want this election stolen from us,” he said. “We do not want this election stolen.”
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.
“As long as there is no irrefutable evidence of fraud, yes,” he told his questioner. “He should — unless there is any refutable evidence to the contrary.”
Mr. Stone is one of the people behind Stop The Steal, a movement of 500 volunteers who plan to stand outside what they believe could be “suspect precincts” on Election Day and conduct their own exit polls to compare against voting machine results.
“In an election in which Donald Trump has made it pretty clear that the Clintons are going to prison, I think they would do anything to make sure they win it, even steal it,” Mr. Stone said.
But, he added, “Trump cannot just lose and say, ‘They stole it.’ He has to have some tangible evidence — and that’s exactly what we’re trying to collect.”
Democrats fear that Mr. Trump’s accusations, in the short term, will lead to voter suppression — and, in the long term, could encourage huge swaths of Americans to view Mrs. Clinton as an illegitimate president if she is elected.
“He’s using phrases like ‘rigged election’ to incite his followers to rig the election by using tactics like voter intimidation, and I don’t think it’s particularly subtle, and I don’t think he cares about the integrity of our elections,” said Stacey Abrams, the Democratic minority leader of the Georgia House of Representatives.
Ms. Abrams, who is African-American and has worked on voting-rights issues, is also the founder of the New Georgia Project, a voter registration and engagement effort in the state. She said Mr. Trump was employing a “voter intimidation model.”
“Just scare them away from the polling place,” she said. “That’s his crude form of voter suppression — not particularly artful, but effective.”
The Clinton campaign is stressing to supporters that they expect voter participation to be higher and easier than in previous elections — but it has also begun recruiting election lawyers to help with voter protection efforts.
“We are prepared for anything in terms of how he chooses to conduct himself in the closing weeks of this campaign, and that includes what is increasingly looking like a scorched-earth approach,” said Brian Fallon, a spokesman for the Clinton campaign.
“He is clearly trying to lay a foundation for challenging the legitimacy of the potential next president, just as he sought to do with the nation’s first African-American president.”
With less than a month until the election, Mr. Trump’s grievances has come fast and furious as he has begun to slip again in the polls.
On Thursday, Mr. Trump also asserted, without offering evidence, that the Obama administration was allowing illegal immigrants to enter the country to vote in November, another example of how he claimed the election was being rigged.
“They’re letting people pour into the country so they can go and vote,” Mr. Trump said at a meeting in New York with the National Border Patrol Council, the union of border patrol agents.
There has been no evidence that the administration is delaying deportations of — or intentionally letting in — immigrants so they can vote. (Illegal immigrants are barred from voting in federal elections.)
Mr. Trump’s claims seem to be resonating among his supporters. At a campaign stop in Iowa on Tuesday, a woman stood up and, her voice quavering, said she feared “voter fraud” before offering a stark call to action to Gov. Mike Pence of Indiana, Mr. Trump’s running mate.
“If Hillary Clinton gets in, I myself, I’m ready for a revolution because we can’t have her in,” the woman said.
Mr. Pence has emerged as Mr. Trump’s most loyal defender. But the call to revolt was a step too far for him. “Yeah, don’t say that,” he said, shaking his right hand as if to try to brush away her comment.
He then tried for a more positive spin: “There’s a revolution coming on November the 8th,” he said. “I promise you.”