President Trump is stepping up his attacks on the integrity of the election system, sowing doubts about the November vote at a time when the pandemic has upended normal balloting and as polls show former Vice President Joseph R. Biden Jr. ahead by large margins.
Having yet to find an effective formula for undercutting Mr. Biden or to lure him into the kinds of culture war fights that the president prefers, Mr. Trump is training more of his fire on the political process in a way that appears intended to give him the option of raising doubts about the legitimacy of the outcome.
Promoting baseless questions about election fraud is nothing new for Mr. Trump. He has hopscotched from saying that President Barack Obama was elected with the help of dead voters to suggesting that undocumented immigrants were voting en masse to claiming that out-of-state voters were bused into New Hampshire in 2016.
But in recent days, Mr. Trump has focused intensive new attacks on voting by mail, as states grapple with the challenge of conducting elections in the middle of surging coronavirus cases in many parts of the country.
On Tuesday, Mr. Trump declared, without offering any evidence, that the 2020 election “will be, in my opinion, the most corrupt election in the history of our country, and we cannot let this happen.”
Mail-in ballots, he said, referring to conspiracy theories, could be stolen from carriers, counterfeited or forged by either forces inside the United States or by “foreign powers who don’t want to see Trump win.”
“There is tremendous evidence of fraud whenever you have mail-in ballots,” Mr. Trump claimed during an appearance in Arizona, a statement that has no basis in the experience of the states that give voters the option of voting by mail.
Mr. Trump has made five dozen false claims about mail balloting since April, as officials in various states began contemplating the need for expanded use of the option amid the pandemic.
About a third of the president’s falsehoods were general warnings about widespread fraud in mail-in voting. Another 11 were specific claims about held-up mail carriers, stolen and forged ballots and dead people voting.
Figures released Wednesday from a New York Times/Siena College survey of battleground-state voters showed that 61 percent strongly or somewhat support allowing all voters to use mail-in ballots if necessary, while 37 percent strongly or somewhat oppose it.
But the poll also suggested that Mr. Trump’s message was getting through to his base: 88 percent of Biden supporters in six battleground states strongly or somewhat support mail-in voting, while 72 percent of Mr. Trump’s supporters strongly or somewhat oppose it.
Justin Clark, the senior counsel to the Trump campaign, defended the president’s words. He said that Mr. Trump was voicing legitimate concerns about how many people would have their hands on ballots with broad mail-in voting, adding, “This is all in the context of a broad Democratic push to greatly expand vote by mail four months before the general election.”
The president’s supporters have already shown that they are taking his assertions to heart. In Michigan, voters began to burn their absentee ballot applications that were sent to them by the state in an act of protest. In Alabama and Kansas, state legislatures have started to pull back from expanded vote-by-mail initiatives.
The president’s attacks on voting by mail stirred widespread concern from current and former election officials and election experts.
“His comments are exceedingly damaging to democracy, to America’s standing in the world, to voters’ confidence in our elections,” said Wendy R. Weiser, the director of the Democracy Program at the Brennan Center for Justice, a nonpartisan think tank. “If you are riling up supporters into a state of anger over the legitimacy of the election, they might actually take steps to try to suppress votes and to undermine the actual legitimate running of the election.”
The unfounded conspiracy theories, some officials said, were no different from the kind of toxic confusion that Russia and other nations have sought to inject into American politics.
“It is misinformation and disinformation, and it’s no different than foreign adversaries and cyberhackers spewing info about an election process,” said Amber McReynolds, a former election official from Colorado and the current chief executive of the National Vote at Home Institute, which promotes voting by mail.
Richard L. Hasen, an election law expert and a professor at the University of California, Irvine, said that unlike the president’s false claims about in-person voting, there have been sporadic problems with mail-in voting, giving Mr. Trump a kernel of truth on which to build an indictment of the entire system.
“The shift has been to mail-in balloting,” Mr. Hasen said of Mr. Trump’s comments, “and this is especially dangerous now because everything about our elections is being upended by the virus.”
The president has directly accused Democrats five times of “rigging” the election through mail-in voting, and has claimed four times that Republicans are at a disadvantage when mail-in ballots are used or are not sent mail-in ballots at all.
Election officials on both sides of the aisle lamented the president’s attacks are making their jobs harder.
Kim Wyman, the Republican secretary of state of Washington State, which conducts its elections almost entirely by mail, pointed to the difficulty of fielding enough workers to count ballots safely during the pandemic or work at the state’s few in-person voting sites.
In California, Alex Padilla, the Democratic secretary of state, said that the president was undermining confidence in the election for his own political benefit.
“What he’s trying to do is absolutely clear: He is not just seeking to undermine the confidence in the election, but confidence in the November election results that he may not like,” Mr. Padilla said.
Mr. Hasen said that Mr. Trump’s denigration of by-mail voting puts into focus scenarios that Democrats are increasingly worried about, like Mr. Trump declaring victory in specific states based on Election Day tallies, when absentee ballot totals could shift the results days later.
Already, 34 states and Washington, D.C., offer no-excuse absentee ballots — including battleground states like Wisconsin, Florida, Michigan, Pennsylvania and Arizona — so even without extra measures introduced in response to the pandemic, voters are likely to utilize mail-in ballots at a much greater rate than previously.
Frank LaRose, the Republican secretary of state of Ohio, who took issue with Mr. Biden when he said that he feared Mr. Trump was trying to “steal” the election, called for an end to such attacks.
“President Trump and Vice President Biden have both questioned the integrity of our elections recently without citing evidence, and they both need to stop it,” Mr. LaRose said in a statement.
Voter fraud, in all forms, is extremely rare, according to numerous independent studies and government reviews. While mail-in voting is less secure than voting in person, fraud remains statistically tiny.
A database of proven election fraud cases maintained by the Heritage Foundation, a conservative think tank, includes 206 cases that involved “fraudulent use of absentee ballots” from 1991 to this year. The Times was unable to find any news reports or instances of mail carriers being held up over mail-in ballots. It would be a federal crime to do so.
California has been a particular focus of Mr. Trump’s ire, featuring in six false attacks, but the battleground states of Michigan and Nevada have earned nods, as well.
He has inaccurately claimed that “anybody in California that’s breathing gets a ballot,” including “people that aren’t citizens, illegals.” State officials will mail ballots to active registered voters only.
He wrongly claimed that Nevada and Michigan had “illegally” sent absentee ballots to voters, and threatened to withhold federal funding should they not rescind the policy, though he did not have the authority to do so.
More recently, Mr. Trump has seized upon the idea that foreign countries could simply print ballots on their own, repeating the claim four times this week without providing evidence.
Election officials and experts have widely rejected this idea as nearly impossible, noting that ballots are printed on very specific stock and often have specific tracking systems like bar codes.
“It’s so much more difficult than a cyberattack. You’d not only have to attack the printer, you’d have to get information from the jurisdiction and get voter registration information,” said Lawrence Norden of the Brennan Center. “You’d have to have some way of mailing the ballots from an address and get the signature of the voter. It would be an exceptionally difficult attack.”