Joe Biden on Thursday night warned donors that President Trump will try to “indirectly steal” the 2020 election by making a case against mail-in ballots, a voting method that many are expected to use to avoid exposure to the novel coronavirus during November’s election.

The presumptive Democratic presidential nominee asked donors attending a virtual event to spread the word that “this president is going to try to indirectly steal the election by arguing that mail-in ballots don’t work.”

Trump will present mail-in ballots as fraud by making the argument that “they’re not real, they’re not fair,” Biden said.

During an appearance in June on “The Daily Show With Trevor Noah,” Biden had previously warned that Trump might “try to steal” the election. He has also said that Trump might try to delay the election and that the president might not leave the White House voluntarily if he does lose.

Biden’s remarks on Thursday offered the most detail about how he believes Trump might unfairly influence the outcome of the presidential contest. Biden’s campaign has mounted an aggressive voter-access effort, including hiring a team of lawyers to challenge any irregularities.

They come as Trump, in recent weeks, has stepped up his rhetoric about the veracity of mail-in voting. At an event last month in Arizona, the president suggested without evidence that the method presents a significant opportunity for fraud.

“Where are they going? Where are these ballots going? Who’s getting them? Who is not getting them? A little section that’s Republican,” Trump said, speaking specifically about California’s vote-by-mail system. “Will they be stolen from mailboxes as they get put in by the mailman? Will they be taken from the mailmen and the mailwomen? Will they be forged? Who is signing them? Who’s signing them? What, are they signed on the kitchen table and sent in? Will they be counterfeited by groups inside our nation?”

Trump added: “Will they be counterfeited, maybe by the millions, by foreign powers who don’t want to see Trump win?”

Matthew Morgan, the general counsel to the Trump campaign, accused Biden of sowing fear.

“Joe Biden is fearmongering and purposely misleading American voters,” Morgan said in a statement emailed to The Washington Post. “The only people trying to fundamentally change the way we vote and make our election system less secure are Democrats.”

Biden made the remarks at a climate-focused fundraising event hosted by actress Julia Louis-Dreyfus, who famously played the first female vice president on the HBO show “Veep.” It was attended by more than 650 people, according to the Biden campaign.

Matt Viser contributed to this report.