The Biden campaign on Thursday said casting ballots by mail is the “most secure form of voting,” as President Trump railed against the practice, warning of widespread voter fraud ahead of the 2020 presidential election.

The president and the Republican Party have been warning for months about possible fraud connected to mail-in voting. The RNC and the Trump campaign have filed lawsuits to hit back against efforts by Democrats to overhaul voting laws in response to the pandemic.

TRUMP SAYS DEMOCRATS' MAIL-IN BALLOT PUSH HOLDING UP CORONAVIRUS STIMULUS NEGOTIATIONS

Trump, during an interview early Thursday with Fox Business’ Maria Bartiromo, slammed universal mail-in voting, warning of “the greatest fraud in history.”

The president went on to say that “there is nothing wrong with getting out and voting” amid the coronavirus pandemic.

“They voted in World War I and in World War II,” Trump said. “They should have voter ID because Democrats scammed the system.”

But Biden campaign Rapid Response Director Andrew Bates on Thursday called the practice safe.

“The President of the United States is sabotaging a basic service that hundreds of millions of people rely upon," said Bates, referring to the U.S. Postal Service, "cutting a critical lifeline for rural economies and for delivery of medicines, because he wants to deprive Americans of their fundamental right to vote safely during the most catastrophic public health crisis in over 100 years -- a crisis so devastatingly worsened by his own failed leadership that we are now the hardest hit country in the world by the coronavirus pandemic.”

"Even Donald Trump's own campaign has endorsed voting by mail and his own administration has conclusively refuted his wild-eyed conspiracy theories about the most secure form of voting,” Bates continued.

He added: “This is an assault on our democracy and economy by a desperate man who's terrified that the American people will force him to confront what he's done everything in his power to escape for months -- responsibility for his own actions.”

Democrats, have been pushing back against the claims by Trump and the GOP for months, say that cases of actual voter fraud are limited and claim that Republicans are trying to suppress voter turnout to improve their chances of winning elections.

Meanwhile, the president blamed Democrats’ push for funding for universal mail-in ballots as one of the reasons for a delay in negotiations on Capitol Hill over a fourth coronavirus stimulus package.

REPUBLICANS WARN MAIL-IN-VOTING RHETORIC COULD BACKFIRE

"It's their fault," Trump said. "They want $3.5 billion for something that's fraudulent ... for the mail-in votes, universal mail-in ballots. They want $25 billion for the post office. They need that money so it can work and they can take these millions and millions of ballots.”

Trump added: "But if they don't get those two items, then they can't have mail-in ballots.”

The president went on to slam the practice, saying that ballots have been “sent to dogs” and "dead people," citing states like Virginia, where he said more than “500,000 phony ballot applications were sent to voters,” and in New York, where mail-in voting caused a weekslong delay in announcing results for some races in the state's primary.

“How would you like to have $3.5 billion for mail-in voting? You know how much money that is? They want $25 billion for the post office because the post office is going to have to go to town to get these ridiculous ballots in,” Trump said.

TRUMP WARNS OF MAIL-IN VOTING 'DISASTER'

He added: “If we don’t make a deal, that means they won’t get the money and they won’t have universal mail-in voting.”

The president went on to cite California, where he said the state is sending out “tens of millions of ballots.”

“Maybe they’ll go to everyone but Republicans,” he said. “We’re challenging it in court. It’s being challenged at many different levels.”

He added: “This will be the greatest fraud in history — this will be almost as fraudulent as Obama spying on my campaign.”

Fox News' Paul Steinhauser contributed to this report.