The allegation, which Trump has been making for months, undercuts the core promise of democracy and sows seeds of doubt among his supporters that
It's another unprecedented claim in modern campaign history that includes a tradition of bipartisan support for the integrity of presidential elections — thus assuring a peaceful transition of power — even from those who lost highly contested battles such as former Vice President Al Gore.
"Of course there is large-scale voter fraud happening on and before election day," Trump tweeted Monday. "Why do Republican leaders deny what is going on? So naive!"
For many in Trump's party, it was another bridge too far. House Speaker Paul D. Ryan and Trump's running mate, Indiana Gov. Mike Pence, had already distanced themselves from the claim. But state election officials were under even more pressure, with their competence as public servants being called into question.
They fought back by describing how elections work and noting that Trump cited no evidence in making his assertion.
"I can say on Twitter I'm a super-model, but that doesn't make it so," said Lynn Bartels, spokeswoman for Colorado Secretary of State Wayne Williams, disputing Trump's claims.
County clerks in her state released an open letter to voters Monday, outlining the security and transparency within the process, adding that "our system is the best in the world and will achieve meaningful and credible outcomes."
Presidential elections are conducted on a state and local basis, not nationally, with more than 8,000 jurisdictions administering elections. And in most of the states seen to varying degrees as presidential battlegrounds, the chief elections officers are Republicans, including in Arizona, Colorado, Georgia, Indiana, Iowa, Michigan, Nebraska, Nevada, New Mexico, Ohio and Utah. They are elected by voters. Most are secretaries of state; Utah's lieutenant governor oversees elections there.
In Florida, the secretary of state is appointed by the state's Republican governor, Rick Scott — a Trump supporter. In North Carolina, the state board of elections has five members, appointed by the governor — currently a Republican. Its current chairman and three out of five members are Republicans.
"I'm a little frustrated when candidates try to distract the public's attention by blaming the technical side of the elections process rather than focus on the meat and potatoes of what Americans really want to hear about," said Iowa Secretary of State Paul Pate, a Republican. "My colleagues in other states, I believe, are of the same opinion."
Nearly half of Trump's supporters lack confidence that ballots will be counted accurately, a Politico/Morning Consult poll released Monday found.
"Perception can become reality," Pate said. "If you keep being told over and over and over that something is hacked even though it isn't, some people might start believing that. So yes, I do take it seriously."
Ohio Secretary of State Jon Husted, a Trump supporter, was equally incensed, telling CNN that Trump's talk was "irresponsible."
"This kind of conversation moves America backward, and it should be dismissed," Husted said. "Don't make people feel despair. Make them feel uplifted."
There are battleground states with some measure of Democratic control over the process, including Minnesota and Missouri, where the chief election officer is a Democrat elected by the voters; Pennsylvania, where the secretary of state was appointed by the state's Democratic governor; and Virginia, where Democratic Gov. Terry McAuliffe — a longtime Clinton ally — appointed each member of that state's three-person elections board.
Trump also said at a rally here Monday night that voter rolls were crowded with the deceased, people in the country illegally or those who were registered in multiple states.
"Your politicians don't tell you about this when they tell you how legitimate all these elections are," he added.
But even the evidence he cited was faulty.
In the case of voter registration deficiencies, Trump pointed to outdated statistics for a record-keeping problem the federal government went on to address with a blue-ribbon panel of election experts that studied and recommended solutions. Some of them, including online voter registrations and data-sharing across state lines, were implemented in several states.
And the Washington Post op-ed on the odds of non-citizens voting that Trump mentioned, written by a pair of researchers, was followed by another piece undermining the methodology of the study, in line with many election experts.
Overall, researchers have found that voter fraud is extremely rare, with only a handful of instances that would be highly unlikely to swing a national election. Two years ago, Loyola Law School professor Justin Levitt, who studies election administration, found 31 credible claims of voter fraud of more than 1 billion ballots cast since 2000.
"It's just not something that I think is realistically possible in the current United States," said Richard L. Hasen, an election law expert who teaches at UC Irvine.
He pointed to the decentralized nature of voting and ballot counting and the ubiquity of observers from both parties as well as neutral groups. Hasen said he believes Trump is merely laying the groundwork for an election loss to avoid being called a loser.
"This has been on the fringe of the Republican Party for years," Hasen said. "Donald Trump is exploiting a population that has been primed to believe that an election can be stolen."
It was not hard to find Trump supporters at his rally here Monday who agreed with him that fraud was likely.
"Wisconsin recently passed a voter ID law. Why doesn't every state have one?" asked Tim McNichol, a 53-year-old computer salesman. "They're letting too many dead people vote."
"I think there's going to be a lot of fraud," he added. "But I still think President Trump's going to win in a landslide."
6:05 p.m: The story was updated with Trump's comments from a rally.