Republican presidential nominee Donald Trump held a campaign rally in Green Bay, Wis. Monday. There, supporters weighed in on recent accusations of voter fraud hampering Election Day results. (Zoeann Murphy/The Washington Post)

Donald Trump’s escalating effort to undermine the presidential election as “rigged” has alarmed government officials administering the vote as well as Democratic and Republican leaders, who are anxiously preparing for the possibility of unrest or even violence on Election Day and for an extended battle over the integrity of the outcome.

Hillary Clinton’s advisers are privately worried that Trump’s calls for his supporters to stand watch at polling places in cities such as Philadelphia for any hint of fraud will result in intimidation tactics that might threaten her supporters and suppress the votes of African Americans and other minorities.

The Democratic nominee’s campaign is recruiting and training hundreds of lawyers to fan out across the country, protecting people’s right to vote and documenting any signs of foul play, according to several people with knowledge of the plans.

“I’m very concerned about this rhetoric,” said former Philadelphia mayor Michael Nutter (D), a Clinton supporter. “All Donald Trump is doing with these outrageous, false scare tactics is to try to diminish voter interest and suppress voter turnout.”

Republican presidential nominee Donald Trump says the election is "rigged" against him, but most of his arguments don't hold up to scrutiny. The Fix's Chris Cillizza explains why. (Peter Stevenson/The Washington Post)

Election officials, including Republican secretaries of state, were also rushing to repudiate Trump’s unsubstantiated claim Monday that fraudulent activity is already underway.

“Any time that your comments draw into question the legitimacy of the elections process, they have crossed the line,” said Ohio Secretary of State Jon Husted, a Republican who supports Trump. “Particularly if you can’t back it up with evidence.”

Trump’s extraordinary attempt to challenge the integrity of the well-regarded U.S. election system comes amid his steep drop in the polls as allegations that he made unwanted sexual advances against multiple women have piled up. Trump denies the allegations and has cast them as part of a far-reaching global conspiracy by Clinton and the media working in concert to defeat him.

While documented cases of voter fraud are extremely rare, the Republican nominee redoubled his effort to raise doubts about the election Monday, challenging his own party to open its eyes to what he says is going on in plain sight.

“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!” Trump wrote on Twitter.

Clinton campaign manager Robby Mook responded Monday that Trump’s campaign is “spiraling” and that Trump is warning of a “rigged” or “stolen” election as a means to distract attention from his problems.

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“He knows he’s losing and trying to blame that on the system,” Mook told reporters. “This is what losers do. And we’re not even going to give it any credit by amplifying it. It’s not true. The system is not rigged. This is probably going to be the easiest, most accessible election in our history.”

Mook predicted a record turnout because the campaign’s analysis of early-voting data shows that voting behavior is exceeding trends from four years ago.

As in every election since 2000, the Clinton campaign and its Democratic allies are organizing a network of volunteer, trained lawyers at key precincts in battleground states. These lawyers can call for immediate help if needed, report intimidation or delays, and move to seek immediate court action or police protection if necessary.

Clinton campaign officials declined to detail their voter protection program. One Democratic lawyer, who spoke on the condition of anonymity, explained the reason: “If you wind up spending a lot of time talking about election fraud and law enforcement, you’re generating a message that can have a very discouraging effect on the electorate. You can wind up lending aid and comfort to the vote-suppression program.”

But people familiar with the plans said there would be a larger-than-normal presence of trained lawyers volunteering at polling places in the battleground states.

The Clinton campaign released a video Monday starring actor Josh Charles, who played lawyer Will Gardner on “The Good Wife,” encouraging lawyers and law students to join the Hillary for America Victory Council, which will staff voter-protection hotlines and send observers to key precincts.

At the White House, press secretary Josh Earnest was asked whether he was concerned about voter fraud and told reporters: “Not at all. Neither is Mike Pence, who is the second-highest-ranking person in the Trump campaign. Neither is Paul Ryan.” Earnest was referring to recent comments from Trump’s running mate and the House speaker’s spokeswoman that played down worries of voter fraud.

Rep. Tom Cole (R-Okla.), a former Oklahoma secretary of state, said he is worried about the alarm bells that Trump is ringing.

“I just don’t believe there is any risk of massive voter fraud in the elections,” Cole said. “. . . It does concern me, because you’ve got a national platform running for president, and you delegitimize the process by which presidents are chosen when you raise doubts.”

GOP leaders, who are fighting to preserve a fragile Senate majority and hold their wider advantage in the House, worry that Trump’s attacks could cast doubt on wins by other Republicans.

Florida Secretary of State Ken Detzner, a Republican, declined through a spokeswoman to be interviewed. In a statement, his office said, “Security during elections and encouraging 100% voter participation in Florida” are Detzner’s “top priorities.”

“The Department of State takes the issue of elections fraud very seriously and diligently works with independent Supervisors of Elections to protect the rights of voters and ensure voter rolls are accurate,” said the statement. “We have many safeguards in place and voters should feel confident that their votes will be counted.”

For months, the Trump campaign has been collecting contact information from supporters who are interested in being election observers.

“Help Me Stop Crooked Hillary From Rigging This Election!” reads the website where people can sign up.

At an August campaign event in Altoona, Pa., Trump urged his crowd not just to vote, but also to go to the polls and look out for signs of illegal activity. The comments alarmed Democrats who worried he was nudging his supporters to go further than the law allows.

“We’re going to watch Pennsylvania. Go down to certain areas and watch and study to make sure other people don’t come in and vote five times,” said Trump. Many interpreted his “certain areas” remark to be a reference to Philadelphia, where there are many African Americans who vote heavily Democratic.

At the same event, Trump said: “We have to call up law enforcement. And we have to have the sheriffs and the police chiefs and everybody watching.”

Roger Stone, a longtime Trump associate, is involved with a group called Stop the Steal, which says it is raising money to recruit poll watchers, conduct exit polling in targeted locations and enlist experts on machine fraud.

Stone wrote an op-ed in the Hill a few days after the Altoona rally spelling out specific concerns: “The issue here is both voter fraud, which is limited but does happen, and election theft through the manipulation of the computerized voting machines.”

Lawrence M. Noble, general counsel of the Campaign Legal Center, said of Trump’s provocations: “When he talks about watching the inner cities, he’s obviously talking about African American voters. When he talks about stopping people from voting five times, what is he asking his people to do? To physically stop people? Even having his people standing around a voting place threatening people is intimidating.”

Benjamin L. Ginsberg, a Republican elections lawyer, said that in most states and jurisdictions, poll watchers need to be credentialed in advance and follow rules, such as remaining a certain distance from polling places.

“Vigilantes can’t go waltzing into polling places and challenging voters,” Ginsberg said. “Every state has provisions that allow both parties to go into the polling place to be sure the vote is fair and accurate. I think it’s important for candidates always to avail themselves of that, both sides. That should certainly be done.”

For many, Trump’s latest rhetoric is a sign that he is coming to terms with the reality of a contest in which he is falling further and further behind Clinton, with time running out to make up ground.

“He wasn’t complaining about all of this during the Republican primaries, when he was winning,” said Nutter, the former Philadelphia mayor. “He thought the system worked perfectly.”

Jenna Johnson, Tom Hamburger and Greg Jaffe contributed to this report.