With concerns mounting over how the country can conduct elections during a pandemic and Democrats pressing for alternatives to in-person voting, President Donald Trump has begun pushing a false argument that has circulated among conservatives for years — that voting by mail is a recipe for fraud.

“Mail ballots, they cheat,” Trump said at the White House on Tuesday afternoon. “Mail ballots are very dangerous for this country because of cheaters. They go collect them. They are fraudulent in many cases. They have to vote. They should have voter ID, by the way.”

The president spoke as Wisconsin voters, many wearing protective masks, were going to the polls on a fraught Election Day, after the Republican-led legislature refused Democratic demands to delay the election and allow for expanded mail-in voting. On Wednesday, Trump claimed again that there was “tremendous potential for voter fraud.”

Here’s a look at the facts on the matter.

All voter fraud is extremely rare.

Studies have shown that all forms of voting fraud are extremely rare in the United States. A national study in 2016 found few credible allegations of fraudulent voting. A panel that Trump charged with investigating election corruption found no real evidence of fraud before he disbanded it in 2018.

Even so, experts say that the mail voting system is more vulnerable to fraud than voting in person.

“What we know can be boiled down to this: Voting fraud in the United States is rare; less rare is fraud using mail ballots,” said Charles Stewart III of the Massachusetts Institute of Technology.

Stewart pointed to several documented voting fraud cases of recent decades involving mail or absentee ballots, most recently the race in North Carolina’s 9th Congressional District in 2018, when an operative who rounded up absentee ballots for the Republican candidate, Mark Harris, was charged with election fraud. The practice is known as ballot harvesting.

“They’re stories; they’re dramatic; they are rare,” said Stewart, a professor of political science who studies the machinations of voting.

The North Carolina case also illustrates the fact that frauds big enough to sway the outcome of an election — those involving campaigns rather than individual voters — will likely be detected, said Michael McDonald, a University of Florida political scientist and elections scholar.

Richard L. Hasen, an elections expert at the University of California, Irvine, School of Law, said such cases were few and far between.

“Election fraud in the United States is very rare, but the most common type of such fraud in the United States involves absentee ballots,” Hasen said. “Sensible rules for handling of absentee ballots make sense, not only to minimize the risk of ballot tampering but to ensure that voters cast valid ballots.”

States that vote entirely by mail see little fraud.

Five states, including the Republican bastion of Utah, now conduct all elections almost entirely by mail. They report very little fraud. The state is among the six states with the highest percentage of mail-in votes in the last election in 2018, all of which had Republican state election supervisors at the time, according to David J. Becker, director of the Center for Election Innovation and Reform.

Colorado, which has 3.5 million registered voters, has been a vote-by-mail state since 2014.

“There’s just very little evidence that there is more than a handful of fraudulent (vote-by-mail) cases across the country in a given election cycle,” said Judd Choate, director of elections in the Colorado Department of State.

Republicans say voting by mail gives Democrats an advantage.

Since the coronavirus outbreak and calls for more widespread mail voting to protect voters, some Republican officials, including Trump, have advanced another argument against the practice: that it will disadvantage Republican candidates.

The president Wednesday tweeted that mail-in voting “for whatever reason, doesn’t work out well for Republicans.” In Georgia, where the secretary of state is mailing all voters absentee ballot requests for the May primary, the state House speaker, David Ralston, a Republican, told a local news site, “This will be extremely devastating to Republicans and conservatives in Georgia,” adding, “This will certainly drive up turnout.”

Even before the coronavirus emerged as a global threat, Democrats had generally favored ways to expand access to voting by mail. Some Republicans have long argued against voting by mail and in favor of tightening voter identification and registration requirements, asserting that easing restrictions invites voter fraud.

Some Democrats have also raised concerns about security.

While Democrats have generally favored mail voting — which works in a very similar way to absentee voting — some have balked at its widespread use because of the reduced level of security when voters are not required to appear at the polls.

Douglas A. Kellner, a co-chair of the New York State Board of Elections, is among the Democrats who have raised flags about voting by mail.

“If you analyze all the steps involved in a mail election you start to see where the weak points are for fraud,” Kellner said, pointing out vulnerabilities in the process, among them the chance that ballots may be intercepted in the mail and forged.

Kellner recalls an absentee ballot harvesting scheme at New York nursing homes during the 1980s that led to changes in the law limiting who may help a nursing home resident fill out a ballot. It was among rules that Gov. Andrew Cuomo suspended last month for the coronavirus pandemic.

The president, vice president and Cabinet members have voted by mail recently.

Trump, while criticizing mail voting, recently acknowledged that in March he voted absentee by mail in his adopted home state of Florida. In the 2018 midterm elections, Trump voted absentee by mail from New York.

Trump is hardly the only administration official to vote by mail. Vice President Mike Pence voted absentee by mail for both the primary and general election in 2018. Alex Azar, the Health and Human Services secretary, also voted absentee in 2018, and Wilbur Ross, the commerce secretary, has voted absentee 15 times in the last 15 years, according to Florida’s voter file. Ross, like Trump, voted absentee for Florida’s primary last month.

Sen, Bernie Sanders last week called upon the nation to prepare to mail vote in the Nov. 3 general election.

“God forbid that this pandemic extends month after month after month after month and that people are unable to vote in the fall,” Sanders said, urging that paper ballots be readied for everyone.