WASHINGTON (AP) -- When Donald Trump cried foul over what he describes as a "rigged" electoral system, his loosely defined claims challenged the essence of America's democratic process and more than 200 years of peaceful transfers of power from one president to the next.
He also added a new element of uncertainty in an extremely heated race: How would Trump and his supporters react to a victory by Democrat Hillary Clinton, whom they view as crooked?
Trump raised concerns Monday after courts rejected tough voter ID rules put in place for the first time in a presidential election in states including North Carolina, Texas and Wisconsin. The rulings cited a risk of disenfranchising the poor, minorities or young people who were less likely to have acceptable IDs - and who are more likely to vote Democratic.
Trump alluded to complaints by Clinton's former Democratic rival about the primary process. "Just like Bernie Sanders, 'I said it was rigged,' well it's rigged here too, believe me," he said.
In all, 17 states are slated to have new restrictions, pending final appeals, even as judges in state after state found that officials could not produce any evidence of widespread fraud involving voter impersonation at the ballot box - the type a photo ID could prevent.
Michael Johns, a co-founder of the conservative Tea Party and a former speechwriter for President George H.W. Bush, told The Associated Press that Trump is bringing taboo issues to the forefront of American political discourse in a way no other candidate has done.
"To have a presidential nominee stand up and say the system is rigged gives (voters) a sense of confidence that their suspicions are on target," he said. Failing to implement tougher voter ID restrictions is "unacceptable and does undermine confidence in the American electoral system."
Yet researchers have found few cases of fraud at the ballot box. A 2014 study by Loyola Law School professor Justin Levitt, who now heads the Justice Department's voting rights unit, found 31 possible instances out of 1 billion votes cast over more than 14 years.
"There is no evidence" that in-person voter fraud "is a factor in elections," said Mary Frances Berry, former chairwoman of the United States Commission on Civil Rights and author of a book on electoral fraud, "Five Dollars and a Pork Chop Sandwich."
Trump "is saying that his voting totals will be overcome by fraudulent Latinos and African-Americans who are permitted to vote, but that kind of fraud, when people pretend to be someone they are not, is minuscule," she added. She said fraud involving bribery or misuse of absentee ballots is more common.
White House spokesman Josh Earnest said Wednesday that President Barack Obama has confidence in the America's electoral process and everybody else should too. He added that fraud claims are often made by supporters of people who lose elections.
Sanders lashed out against the Democratic primary system earlier this year, claiming that it favored Clinton. He was somewhat vindicated when, last month, leaked Democratic National Committee emails revealed that party officials did prefer Clinton. But his complaints were about the candidate-selection process, not ballot box fraud.
Claims of foul play in presidential politics are almost as old as the union. In recent times, voter fraud allegations received the most attention in 2000 when Democrat Al Gore lost to George W. Bush after a painstaking 36-day recount in Florida. The Supreme Court ended a statewide hand recount of "undervotes" -ballots that did not register as a vote. Several post-mortem reviews concluded that had the undervotes been counted, Gore would have won. Sen. John McCain said Sen. Barack Obama was trying to "steal" the election through groups like low-income advocacy group, Acorn.
In Chicago, prosecutors are examining allegations that campaign workers paid people during the March primary to vote for a legislative candidate. Charges haven't been filed, but the case fed into Chicago's "vote early and often" reputation for election fraud.
Roger Stone, one of Trump's closest confidants, predicts widespread voter fraud. He said Trump needs to talk about it constantly.
If Clinton wins, the inauguration will prompt "civil disobedience, not violence, but it will be a bloodbath," he said in an interview with the conservative website Breitbart. "No, we will not stand for it. We will not stand for it."
Trump's claims of a rigged system have fanned flames in a critical week for his campaign in which he's bounced from one controversy to the next. Frustration among Republican leaders mounted Wednesday following a series of startling statements from the GOP nominee, including his refusal to endorse House Speaker Paul Ryan's re-election.
Michael Waldman, president of the Brennan Center for Justice, a nonpartisan public policy and law institute, said courts have repeatedly found little evidence of voter fraud and significant evidence that restrictive laws deter legitimate voters.
"Statistically you are more likely to be hit by lightning than to commit in-person voter fraud," he said. "It's a scare tactic and it's unfortunate."
Associated Press writers Hope Yen and Josh Lederman in Washington, Jon Lemire in Daytona Beach, Florida, and Sophia Tareen in Chicago contributed to this report.