During a meeting with representatives from the National Border Patrol Council, Republican presidential nominee Donald Trump said the United States is “letting people pour into the country so they can go and vote.” (The Washington Post)

Donald Trump suggested without evidence Friday that the Obama administration was letting illegal immigrants into the country to vote — part of a series of unsubstantiated complaints by the GOP nominee that the election is “rigged” against him and that his backers should monitor polling locations in “certain areas.”

Trump’s allegations were a dramatic escalation of the usual partisan warfare over ballot access issues and came as Florida Gov. Rick Scott (R) denied a request by Hillary Clinton’s campaign to extend voter registration because of Hurricane Matthew. The storm caused the extension of voter registration deadlines in South Carolina, while officials in Georgia have urged residents in storm-affected areas to register online instead of going to registration centers.

In Nevada, state Democrats also threatened Friday to file suit if the voter registration deadline is not extended beyond Saturday to comply with a federal law requiring deadlines to fall within the 30 days before an election.

Voting experts say they are increasingly troubled by Trump’s tone amid fresh strain on voting systems nationwide, warning that his comments could undermine trust in an election system renowned as largely efficient and free of corruption.

“We’ve never seen anything like this coming from a presidential candidate,” said Richard L. Hasen, a law and political science professor at the University of California at Irvine who is the co-author of a leading casebook on election law.

Hasen said Trump’s comments about undocumented immigrants illegally voting is a continuation of his “irresponsible” pushing of the discredited idea that voter fraud is rampant and undetected, especially in poor and minority areas. Hasen said voter impersonation fraud is rare and that in his years of research, he has yet to find a single election where such cases compromised the results.

“It just doesn’t happen in the United States,” he said.

Daniel A. Smith, a political science professor at the University of Florida, added that Trump is “trying to undermine the election system, which is very decentralized, which has very good people.”

“If anything, it’s putting a damper on the ability to register and creating some paranoia that is unfounded,” Smith added.

Trump’s latest comments came during an event on border security held on the 25th floor of Trump Tower on Friday.

Art Del Cueto of the National Border Patrol Council, which represents U.S. Border Patrol agents and has endorsed Trump, claimed that agents were instructed not to deport undocumented immigrants with criminal records.

“Why?” Trump asked.

“So they can go ahead and vote before the election,” Del Cueto responded.

Trump replied: “Big statement, fellas.” Motioning to the small group of reporters, he added, “You’re not going to write it. That’s huge. But they’re letting people pour into the country so they can go and vote.”

The union later clarified that U.S. Citizenship and Immigration Services has authorized extra overtime for employees processing a higher volume of U.S. citizenship applications before Election Day. It also claimed in a statement to have “text messages from upper level managers” saying that criminal prosecution cases were being put on hold so that immigration judges could make rulings on citizenship applications.

The Obama administration has focused on targeting for deportation undocumented immigrants who have committed felonies. Marsha Catron, a spokeswoman for the Department of Homeland Security, said that “our borders are not open to illegal migration. We must and we will enforce the law in accordance with our enforcement priorities. Our actions reflect that commitment.”

Only U.S. citizens can vote in federal elections. To apply for citizenship, an individual must be a legal resident for at least three to five years and meet other requirements.

Trump has repeatedly warned his supporters that he might lose the election because the system is “rigged” against him, and his campaign is recruiting election observers.

During rallies held in white suburbs of diverse cities, Trump has often urged his followers to go to neighborhoods other than their own and “watch” the voters there. Many voting rights activists have accused Trump of encouraging voter intimidation.

“You’ve been reading the same stories as I’ve been reading, so go to your place and vote, and then go pick some other place, and go sit there with your friends and make sure it’s on the up and up,” Trump said at a rally in the Detroit suburbs Sept. 30.

Because of Hurricane Matthew, South Carolina Gov. Nikki Haley (R) announced that the state plans to accept voter applications online and by email until Sunday and by mail until Tuesday.

The Clinton campaign began pressing Florida election officials to do the same.

“Our hope would be that a little more time will be given,” Clinton campaign manager Robby Mook told reporters Thursday. “We certainly expect that the governor and local officials will make that possible.”

But Scott declined, saying that “everybody has had a lot of time to register. On top of that, we’ve got lots of opportunities to vote: Early voting, absentee voting and Election Day. So I don’t intend to make any changes.”

Scott’s decision could keep thousands of Floridians from participating in the election, according to Smith, who is studying the voting habits of people who register to vote in the later months of an election season.

About 50,000 individuals successfully registered to vote in the final five days of Florida’s registration period in 2012, according to state data analyzed by Smith. Thirty-nine percent of registrants were Democrats; 39 percent independents; and 22 percent Republicans. About 47 percent of the registrants were white; about 19 percent Hispanic; and 18 percent black.

Three out of four people who registered in the last five days of the registration period ended up voting, Smith said. “While they may have been last-minute in terms of registration, they certainly had the election on their mind.”

Hasen said problems with registrations could lead to even bigger problems on Election Day and noted that after Superstorm Sandy in 2012, some jurisdictions in New Jersey relaxed the voting rules in ways that violated state law and might have even led to some fraudulent voting.

“Just imagine if any of this happens in Florida after Matthew,” Hasen wrote in a column for Slate this week. “We already have Donald Trump telling voters that the election is rigged. Any attempt to try to accommodate, or fail to accommodate, voters will be second-guessed, challenged, and likely litigated.”

Sean Sullivan in New York contributed to this report.