As Donald Trump's campaign falters, his warnings that the presidential contest will be rigged have become a focus of his pitch to voters.

Historians say Trump's sustained effort to call the process into question has no close parallel in past elections. And some are increasingly worried that his claims — for which he's offered no real evidence — could leave many of his supporters unwilling to accept the election results, potentially triggering violence and dangerously undermining faith in American democracy.

Day after day — at rallies, in interviews and on Twitter — Trump and several top backers have hammered the message that a victory for Hillary Clinton would be illegitimate. Trump has frequently suggested that widespread voter fraud will swing the election, and he has urged his supporters to closely monitor the voting process.

In a tweet Monday, he declared that there's "large-scale voter fraud happening on and before election day." In fact, numerous studies have shown that in-person voter fraud is vanishingly rare.

Of course there is large scale voter fraud happening on and before election day. Why do Republican leaders deny what is going on? So naive! — Donald J. Trump (@realDonaldTrump) October 17, 2016

In August, Trump told a Pennsylvania crowd that the "only way" he could lose the state is "if cheating goes on." Trump's vice presidential running mate, Indiana Gov. Mike Pence, echoed those claims Monday in Ohio, declaring: "Voter fraud cannot be tolerated by anyone in this nation."

Trump is hardly the first prominent Republican to issue dire warnings about voter fraud. In 2008, Sen. John McCain of Arizona alleged in a presidential debate that the voter registration group ACORN was "on the verge of maybe perpetrating one of the greatest frauds in voter history in this country, maybe destroying the fabric of democracy."

But as he has slipped in the polls, Trump has gone further, making his claims a central facet of his campaign — to the point where even some Republican leaders, including House Speaker Paul Ryan, have repudiated them. And he has broadened his case, charging that the contest is being rigged not just through fraud but also by the media, which he says favor his opponent.

He has also suggested that Clinton worked with the Democratic National Committee to steal her party's nomination from Sen. Bernie Sanders of Vermont.

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Laura Belmonte, a history professor at Oklahoma State University, said that although there have been disputed elections and claims of illegal voting in the past, Trump's systematic effort to question the process in advance is new.

"I really can't think of another precedent where this rhetoric has been used so vigorously prior to the election," Belmonte said. "So the calls for poll watchers and the not-so-veiled threatening discourse — I really don't think have an analogue."

Belmonte added that most losers of close presidential elections have conceded defeat and called for the nation to unify, which has helped to maintain public faith in the system.

In 1877, Democrat Samuel Tilden gave a speech in which he continued to claim that he was the rightful winner of the previous year's election, but he nonetheless called on his supporters to "be of good cheer," adding: "The Republic will survive."

Still, Belmonte said, some of them went on to refer to Tilden's opponent, President Rutherford B. Hayes," as "Rutherford B. Fraud."

More recent losers of disputed elections have been even clearer. Richard Nixon pledged his "wholehearted support" for John F. Kennedy in 1960, even as some Nixon supporters alleged fraud by Democrats in crucial Illinois.

In 2000, Vice President Al Gore — who had won the popular vote and might have won a majority of electoral votes were it not for a purge of Florida's voter rolls while opponent's brother was governor — promised to "bring Americans together" after the Supreme Court decided the election for George W. Bush.

Whether Trump would respond to defeat in the same way is very much an open question. He pledged at the first presidential debate that he'd "absolutely" support Clinton if she won. But he has since hedged, telling The New York Times later that week: "We're going to have to see. We're going to see what happens."

That has some election experts worrying that America's long tradition of peacefully transferring power could be at risk.

"One of the things we take for granted is that, even in tumultuous times when elections are hard fought, the losers concede the election and embrace the process, even if things did not go well," the election law scholar Rick Hasen, a professor of law and political science at the University of California-Irvine, wrote after Trump's "we're going to have to see" comments.

"Donald Trump threatens this peace," Hasen wrote.

Evidence is mounting that Trump's broad, sustained attack has already had an impact:

A Politico poll released Monday found 41 percent of voters — including 73 percent of Republicans — fear that the election could be stolen.

An Associated Press poll found that half of respondents who have a favorable opinion of Trump have little to no confidence that votes will be counted fairly.

And a Survey Monkey poll conducted with Nathaniel Persily of Stanford Law School earlier this month, found that 40 percent of respondents said they'd lost faith in American democracy, with Trump supporters saying so at significantly higher rates than Clinton backers.

Meanwhile, the rhetoric from Trump supporters is growing apocalyptic. Milwaukee County, Wis., Sheriff David Clarke, a prominent Trump surrogate on law and order issues, called in a recenttweet for "pitchforks and torches."

It's incredible that our institutions of gov, WH, Congress, DOJ, and big media are corrupt & all we do is bitch. Pitchforks and torches time pic.twitter.com/8G5G0daGVN — David A. Clarke, Jr. (@SheriffClarke) October 15, 2016

"We're going to have a revolution and take them out of office if that's what it takes" if Clinton wins, a Trump backer told The Boston Globe at a rally in Ohio. "There's going to be a lot of bloodshed. But that's what it's going to take."

Last week, two armed Virginia men supporting Trump stood for hours outside a Democratic campaign office to "protest" Clinton. The action was legal, but it appeared designed to intimidate.

One reason Trump's allegations may be resonating is that they're of a piece with the broader thrust of his campaign. Since he kicked off his campaign last year, Trump has been telling voters that they were being sold out by corrupt elites and special interests who benefit from policies like free trade and open borders.

Saying the election is being rigged is, in some ways, only an extension of that outlook.

But Belmonte warned that Trump's rhetoric could have a lasting impact.

"I don't see how we put the toothpaste back in the tube with a lot of this very violent, super-heated rhetoric that's been in the public discourse this year," Belmonte said. "That's not going to just flip off on November 9th."