Mr. Trump last week hired as his campaign chief Stephen K. Bannon, the executive chairman of Breitbart, a conservative news website that has frequently given voice to Mr. Trumpâ??s claims of a manipulated process, holding forth on perceived voter fraud and â??propaganda pollsâ? showing Mrs. Clinton ahead.
And on Friday, Mr. Trump released his first campaign ad, focused on immigration, featuring an image of a polling site with the word â??riggedâ? flashing onscreen less than two seconds after the spot begins.
Election law officials have expressed concerns that Mr. Trumpâ??s incendiary words will create a self-fulfilling prophecy, all but ensuring there will be claims of fraud from his poll watchers and a delegitimization of the election results should Mrs. Clinton win.
â??It went from being laughable to be what I consider to be dangerous,â? said Richard L. Hasen, a professor and election law expert at the University of California, Irvine, School of Law.
Mr. Hasen said that while it initially seemed that Mr. Trump was merely seeking an early scapegoat for a possible loss, his language had taken a darker turn. A Pew Research Center survey released last week showed that 51 percent of Mr. Trumpâ??s supporters have little or no confidence in the accuracy of the vote count nationally, a dramatic change from supporters of the Republican nominee in 2004 and 2008.
Concerns about a rigged election have sometimes gnawed at American politics but were most pronounced after the 2000 presidential race. In that race, the popular vote winner, Al Gore, a Democrat and a former vice president, was separated from George W. Bush, a Republican, by a few hundred votes in Florida. The United States Supreme Court ultimately settled the election in Mr. Bushâ??s favor.
After the 2012 presidential election, some conservatives made claims of voter fraud in Pennsylvania that were never substantiated. Mark Braden, a Republican election lawyer, said that while there have been cases of voter fraud over decades, â??the election system in the United States generally works extremely well and fraud, although real, is modest.â?
Demonstrable episodes of widespread individual fraud have been hard to come by. According to a study by the nonpartisan Brennan Center in 2007, â??by any measure, voter fraud is extraordinarily rare.â?
Mr. Trumpâ??s assertions echo a theme he has increasingly pursued in recent weeks: that the political system is â??riggedâ? â?? he points to the lack of charges against Mrs. Clinton over the use of a private email server while serving as secretary of state â?? and that the news media is tilting its coverage to benefit Democrats.
Credit Mark Makela for The New York Times
But his suggestions that voting itself will be tainted could have a longer-term resonance. Some Republicans have since 2000 pointed to an increased use of electronic voting machines to suggest that results could be subject to tampering. Roger J. Stone Jr., an informal adviser to Mr. Trump, wrote in The Hill that electronic voting machines could be â??manipulated.â?
â??We are now living in a fake reality of constructed data and phony polls,â? he added.
Marc Elias, the main counsel to Mrs. Clintonâ??s campaign and a lawyer involved in cases against a string of strict voter-identification laws in states such as North Carolina and Virginia in recent years, called such talk fear-mongering aimed at depressing minority turnout.
â??Itâ??s a sad day when Donald Trump and the Republicans have to rely on scaring people out of voting to try to achieve their electoral aims,â? Mr. Elias said.
Feeding into Mr. Trumpâ??s expressions of concerns is a federal appeals courtâ??s July ruling striking down North Carolinaâ??s voter identification law because it targeted African-Americans. North Carolina is a swing state that was, until recently, dependably Republican, but polls have shown Mrs. Clinton with an edge there.
After years of conspiracy theories about President Obamaâ??s birthplace â?? propagated by Mr. Trump, among others who have sought to delegitimize the presidentâ??s rise to power â?? Democrats fear the voting claims could resonate among opponents of Mrs. Clinton long after Election Day, should she win.
Last week, Mrs. Clinton took her campaign to the heart of a neighborhood that election conspiracy theorists have viewed suspiciously: West Philadelphia, a predominantly black area where, in 2012, Mr. Obama captured 100 percent of the vote in some precincts.
Speaking last Tuesday at a high school gymnasium â?? across from a strip of moldering rowhouses with windows that occasionally featured Clinton signs â?? Mrs. Clinton steered clear of Mr. Trumpâ??s claims, at least directly, making the case that relentless voter registration efforts were the best defense against his tactics.
â??We know what we need to do,â? she told the crowd. â??The question is whether we will do it.â?
Mr. Trumpâ??s charge of prospective fraud has rankled black voters and Democratic leaders, by turns calling to mind a painful history of racially charged voter intimidation, and inspiring bemusement at the notion that Mr. Trump thinks he might be able to win the support of minorities on the merits â?? if only he could root out the chicanery.
â??If he thinks that Obama winning North Philly and West Philly was rigged, he should come out himself and speak to every voter and see exactly who they voted for,â? said James Walton, 30, from North Philadelphia, another largely black area of the city.
Darrell L. Clarke, Philadelphiaâ??s City Council president, said the racial overtones of Mr. Trumpâ??s remarks were clear.
â??When you talk about â??certain areasâ?? in Pennsylvania, we all know what that means,â? he said. â??Heâ??s talking about Philadelphia and some of the urban areas.â?
The Trump campaign recently started a website urging people to sign up as election watchers. All campaigns bring on poll watchers, but they are required to go through extensive training about what crosses the line into intimidation.
The Republican National Committee has been operating under a consent decree for more than three decades, after claims that members of the committee intimidated minority voters at the polls in the 1970s and 1980s. Mr. Trumpâ??s campaign, according to committee officials, is not bound by that document, despite the intermingling of their resources.
The consent decree is set to expire soon, but not before the election. Republicans have fretted that Mr. Trumpâ??s bombast would invite Democrats to fight to keep it in place.
Mr. Braden, the Republican election lawyer, said that sweeping talk about fraud could backfire in tight Senate races in which Republicans end up ahead by a slim margin.
â??From my perspective, no, it is not helping,â? he said. â??A more measured, thoughtful discussion of the issue, which is worth discussing, would be helpful.â?
â??But that,â? he added, â??does not seem to be his style.â?