WASHINGTON - President Donald Trump and Attorney General Jeff Sessions on Monday issued strong warnings about the threat of voter fraud in Tuesday's elections, echoing the president's baseless claims that massive voter fraud marred his 2016 election and prompting accusations that his administration is trying to intimidate voters.

In a tweet early Monday, Trump said that law enforcement has been "strongly notified" to watch for "ILLEGAL VOTING." He promised that anyone caught voting improperly would be subjected to "Maximum Criminal Penalties." Sessions, in a statement laying out the Justice Department's plans to monitor ballot access on Election Day, said "fraud in the voting process will not be tolerated. Fraud also corrupts the integrity of the ballot."

In remarks to reporters on his way to a campaign rally in Cleveland, Trump also falsely claimed that voter fraud is commonplace.

"Just take a look," he said. "All you have to do is go around, take a look at what's happened over the years, and you'll see. There are a lot of people - a lot of people - my opinion, and based on proof - that try and get in illegally and actually vote illegally. So we just want to let them know that there will be prosecutions at the highest level."

There is no evidence of widespread voter fraud in the United States. Trump formed a commission to study the issue shortly after he took office that was disbanded without finding evidence of fraud after states refused to turn over voter data.

Voting rights advocates denounced Trump's remarks as a blatant attempt to intimidate voters on the eve of Election Day - and part of a pattern among Republicans, they said, to curtail voting access with strict rules that disproportionately affect voters of color who tend to vote Democratic.

"I find this kind of conduct incredibly anti-patriotic," said Kristen Clarke, who leads the Lawyers' Committee for Civil Rights Under Law, a voting rights group that has successfully challenged several new voting restrictions across the country this year. "At a time when we need our White House and Justice Department speaking out against the relentless campaign of voter suppression in this election cycle, it defies reason."

Accusations of voter fraud and voter suppression have roared to the forefront in several closely contested races this year, raising the possibility of recounts and disputed results among dozens of races for House, Senate and governor.

Anticipating possible problems at the polls, political parties, interest groups and voting rights organizations have organized "war rooms" to watch Tuesday's elections unfold and recruited thousands of volunteer lawyers to monitor precincts across the country. In his statement, Sessions said the Justice Department will follow its usual protocol of sending monitors across the country to protect against voter suppression, intimidation and discrimination; this year, staff will travel to 35 jurisdictions in 19 states to monitor compliance with voting laws.

In past years, Justice Department officials have not listed voter fraud as a top concern when announcing the deployment of election monitors, as Sessions did Monday.

"It's indicative of a pattern with this administration," said David Vance, a spokesman for Common Cause, a civil rights group that helped recruit 6,500 volunteers to monitor polling locations across the country Tuesday. "It's an effort to intimidate voters and keep them away from the polls and try to dictate which voters will turn out and which voters won't. It flies in the face of what the DOJ has done traditionally to protect voters."

Homeland Security officials also urged vigilance about a different kind of threat to election security: interference by foreign actors, particularly Russia. They "continue to try to influence public sentiment and voter perceptions through actions intended to sow discord," such as spreading disinformation on social media, officials said in a statement.

Nowhere has the debate over voting rights been more acrimonious than in Georgia, where Republican Brian Kemp, a champion of voting restrictions, is running for governor against Democrat Stacey Abrams, who would be the first-ever black female governor in the nation.

Kemp, who as secretary of state is also in charge of running Tuesday's elections, over the weekend accused the state Democratic Party of trying to hack into the state election system. Emails from the party, however, show that party officials were alerted to a possible vulnerability in the state system and forwarded the tip to cybersecurity officials, who in turn forwarded it to lawyers for Kemp as well as the FBI.

On Monday, computer experts and lawyers involved with the episode said they were stunned that Kemp had turned an effort to alert his office about a security vulnerability into a political attack. "It's obvious to me that they're shooting the messenger," said Logan Lamb, an Atlanta-based cybersecurity researcher who reviewed the emails over the weekend.

A spokeswoman for Kemp said Monday that his office has referred the matter to the Georgia Bureau of Investigation.

Kemp has battled Abrams over voting issues for years. In 2014, he accused the New Georgia Project, a voter-registration group that Abrams founded to register eligible people of color, of fraud, but his investigation produced no evidence of wrongdoing. This year, he accused Abrams of encouraging undocumented immigrants to vote, a charge she denies. This week he circulated the accusation in an automated telephone recording.

Kemp also championed strict new voting rules that were partially blocked in a pair of court rulings in recent weeks. One of the laws, requiring personal information on voter registration applications to exactly match other government records, disproportionately affected people of color.

Kemp also tweeted an article Monday from Breitbart, a conservative news outlet that regularly publishes right-wing conspiracy theories, claiming that "armed Blank Panthers" support Abrams. The racially charged article featured photographs of black men carrying guns and holding Abrams signs.

Accusations and anxiety surfaced in other corners of the nation, too. A California group called the Voting Rights Task Force recruited volunteers on Twitter on Monday to monitor election returns on county websites every 15 minutes "to make sure vote totals don't go DOWN and to capture it in screenshots if they do."

In the Central Valley of California, Democrat Andrew Janz, who is challenging Rep. Devin Nunes, the chairman of the House Intelligence Committee, accused local election officials of changing his polling location without notice - and he urged his Twitter followers to check their polling places before heading out to vote Tuesday.

It turns out that Janz's polling location had not been changed but that a state website had given him incorrect information, according to the Fresno Bee.

In North Dakota, advocates continued to monitor a restrictive voter ID law requiring voters to provide identification displaying a residential address, which they say will make it harder for Native Americans to cast ballots because they are less likely to have the necessary documents.

Meanwhile, voting rights were on the ballot in some places. In North Carolina, voters will decide Tuesday whether to require a photo ID to vote. In Kansas, the fiery secretary of state, Kris Kobach, is in a tough gubernatorial race in which his false claims of rampant voter fraud have become a dominant issue. Several House Democrats have promised that new voter protections, including restoring provisions of the Voting Rights Act struck down by the Supreme Court, will be among their top priorities.

One bright spot for voting rights advocates has been the boost in turnout among early voters logged across the nation. As of Saturday, the number of people voting early had outpaced that of the 2014 midterm elections in 28 states, according to data compiled by Michael McDonald, a political-science professor at the University of Florida. In two additional states, Texas and Nevada, early voting was on track to surpass the entire vote count four years ago, he said.

Said Vance, of Common Cause: "To turn out and make yourself heard is the primary weapon to combat these attempts to suppress the vote."

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The Washington Post's Devlin Barrett and Cleve Wootson contributed to this report.