Biden has attempted to combat Trump’s onslaught by pointing out that Trump himself has voted-by-mail in Florida from the White House.
Some elections experts argue Trump’s offensive against mail-in voting is a voter suppression tool, which could lay the "groundwork for contesting a close election if he loses, " wrote Rick Hasen, a professor of law and political science at the University of California, Irvine, and the author of "Election Meltdown: Dirty Tricks, Distrust and the Threat to American Democracy."
"The most benign explanation for Mr. Trump’s obsessive focus on mail-in balloting is that he is looking for an excuse for a possible loss to his Democratic opponent, Joe Biden, in November. The less benign explanation is that he is seeking to sow chaos to drive down turnout and undermine the legitimacy of the election," wrote Hasen.
At the first presidential debate on Tuesday at Case Western Reserve University in Cleveland, Ohio, the two rivals are set to spar over the integrity of the election, one of the topics previewed by the Commission on Presidential Debates.
Here's what we know about the candidates' histories with and stances on the issue:
The Russia question
Even before this year’s election, questions about the integrity of the 2016 race dogged Trump throughout his presidency after intelligence officials disclosed an extensive influence campaign directed by Russian President Vladimir Putin himself to aid Trump.
A bipartisan Senate panel this year affirmed the intelligence community's assertions, rejecting the notion that the intelligence community was biased against Trump in its approach.
The disclosure of the influence campaign and an investigation into whether his team was colluding with Russia cast a "cloud," as Trump described it, over the early years of his first term.
Even after the probe by Robert Mueller, the special counsel appointed to investigate Russian meddling, found that it could not substantiate the collusion or conspiracy allegations, the issue remained a sore point with the president. Losing the popular vote by millions and winning some states by thin margins kept Trump on the defensive with critics, who continued to assert that his victory might have been undeserved.
Trump repeatedly dismissed the suggestion of Russian playing a role in putting him in the White House, and cast his team's involvement with Russian operatives as a "hoax," while blaming the Obama administration, including Biden, for being asleep at the switch.
Asked about Trump doubting Russia’s interference in 2016, Biden said in an interview with CBS in October 2019, "He is an idiot — in terms of saying that. Everybody knows this. Everybody knows it. Nobody doubts it."
Ahead of the 2018 midterms, national security officials again warned that Russia was at it again. In the wake of the midterms, the intelligence community reported that while the vote had not been compromised, influence campaigns, including from Russia and Iran, persisted. The review stemmed from a September 2018 executive order mandating regular election assessments.
States' turn to mail-in voting for November
This year, election security has emerged at the center of the 2020 presidential race in part by Trump's own making -- he has relentlessly assailed the use of the mail-in voting with unfounded suggestions of widespread fraud.
"The only way we're going to lose this election is if the election is rigged," he said during a visit to Wisconsin in August. Trump made the same assertion in the lead-up to the 2016 race.
For months leading into this fall, Trump has sought to sow distrust in the voting practice, including baselessly suggesting that foreign actors could take advantage, just as the country is relying on it most.
Before November, at least 32 states, plus Washington, D.C. have adjusted their election blueprints to expand options for voters and to make voting more accessible due to COVID-19.
Trump ramped up his warnings of widespread fraud last week, even declining to commit to a peaceful transition of power if he were to lose the election, and pushing his belief that mail-in ballots will be the problem. "Get rid of the ballots and you'll have a very peaceful -- there won't be a transfer," he said.
"What country are we in?" Biden said, when a reporter asked him to respond. " I don’t know what to say about it, but it doesn’t surprise me."
In a later interview with MSNBC, Biden said the American people will be heard.
"Every vote in this country is gonna be heard, and they’ll not be stopped," he said. "I am confident all the irresponsible, outrageous attacks on voting -- we’ll have an election in this country that we always have had. And he’ll leave."
The president’s efforts come as his own campaign and state Republican parties encourage supporters to use mail-in voting through ads, online posts, and robocalls -- and as his own intelligence officials refute his narrative.
"We have not seen, historically, any kind of coordinated national voter fraud effort in a major election, whether it's by mail or otherwise," FBI Director Christopher Wray said before Congress Thursday.
Biden, for his part, explicitly backed the nationwide push to expand voting-by-mail in response to the pandemic, even calling on Congress to provide more funding to aid states in their efforts, while casting Trump's strategy as "un-American."
His campaign is also building out its legal operation in anticipation of a major fight over voting this fall, bringing on two former U.S. solicitors general and other top attorneys to oversee what could devolve into a fierce battle over the November's results.
Further stoking concerns over the integrity of the election regarding mail-in voting is the Trump administration’s assault on one of the country’s most storied institutions, the United States Postal Service.
The USPS implemented controversial cost-cutting measures under Trump-installed postmaster general Louis DeJoy, which some critics argued could have adverse impacts on the election by slowing mail delivery. Those changes were halted by a federal judge earlier this month.
House Democrats passed a bill in August that would've allocated an additional $25 billion in funding to USPS, a cash injection the agency's board of governors -- whose Senate-confirmed members were all appointed by Trump -- requested from Congress back in early April because of anticipated losses.
Trump has attempted to use this as evidence that USPS won't be able to handle the increased mail-in voting, asserting, "they don't have the money to do the universal mail-in voting. So therefore, they can't do it," Trump said on Aug. 12. But DeJoy rebuffed Trump, saying on Friday, "we’re equipped to do it and we’re going to deliver the ballots."
Trump claims foreign governments interfering with mail-in voting
The president has also raised the possibility of foreign governments trying to interfere in the presidential election by taking advantage of the increase in mail-in voting, an unproven claim that he first made after Attorney General Bill Barr suggested that foreign countries could do this, and that it would be hard for U.S. election officials to detect it, during an interview on Fox News in June.
"The biggest risk that we have is mail-in ballots, because with a mail-in ballot -- well, universal mail-in ballots, it is a much easier thing for a foreign power, whether it's Russia, China, Iran, North Korea...it's much easier for them to forge ballots and send them in," Trump said in early August. "It's much easier for them to cheat with universal mail-in ballots."
However, election officials, and the FBI, have disputed this scenario, citing the safeguards and specificities in place -- like ballots being printed on special paper or having unique barcodes on them -- that make this highly unlikely to happen, never mind happen undetected.
Specter of Russian influence and interference once again
Russia’s attempts to meddle extend beyond mail voting. In early September, ABC News reported that the Department of Homeland Security withheld publication of an intelligence bulletin in early July warning law enforcement agencies of a Russian scheme to attack Biden’s mental fitness, according to internal emails and a draft of the document obtained by ABC News.
The bulletin detailing Russia’s campaign against Biden’s mental health again echoes Trump’s own rhetoric -- and indicates the most specific example to date of Moscow’s intentions to influence November’s election.
There is no evidence to suggest that Biden's not mentally fit to be president, and both he and his campaign have pushed against this narrative from Trump.
In June of last year, Trump even said in an exclusive interview with ABC News he would "listen" if foreign governments offered dirt on political opponents.
"I think you might want to listen, there isn't anything wrong with listening," Trump told ABC News Chief Anchor George Stephanopoulos. "If somebody called from a country, Norway, [and said] ‘we have information on your opponent' -- oh, I think I'd want to hear it."
Other foreign powers a worry
But Russia isn’t the only foreign country seeking to sway America’s vote. Back in August, a public statement issued by the U.S. intelligence community revealed that three of the nation’s top adversaries are actively attempting to meddle in the presidential race, including efforts by Russia to advance Trump's reelection efforts.
Intelligence officials also concluded that "China prefers that President Trump – whom Beijing sees as unpredictable – does not win reelection." Iran, it found, "seeks to undermine U.S. democratic institutions, President Trump, and to divide the country in advance of the 2020 elections."
Asked about the statement, Trump deflected, turning the question back on the reporter.
"We're gonna look at that very closely, but you started off with Russia. Why don't you start off with China? We think China's maybe a bigger threat," Trump said during a press conference at his golf club in Bedminster, New Jersey.
Trump has frequently downplayed intelligence showing Russia’s aim to aid his campaign in 2016, while Biden excoriated his rival’s administration for failing to take action to prevent foreign election interference, and said that he is putting the Kremlin and other foreign governments "on notice."
"If elected president, I will treat foreign interference in our election as an adversarial act that significantly affects the relationship between the United States and the interfering nation's government," Biden wrote as part of a lengthy statement.
However, the last time Biden was in the White House, the Obama administration did not publicly comment on Russia’s efforts until October of 2016, when they accused the foreign government of hacking political institutions, including the Department of Homeland Security, and interfering in the election. Former DHS Secretary Jeh Johnson defended that delay in June 2017, saying that there has to be consideration given to protecting sources and methods, and also, that “many would criticize us for perhaps taking sides in the election.”
But according to a bipartisan conclusion reached by members of the Senate Select Committee on Intelligence in June 2018, the Obama-Biden administration did not do enough to combat Russia’s election meddling during the 2016 campaign. Vice Chairman Mark Warner, D-Va., described officials as being “caught flat-footed.”
The overt differences between Biden and Trump on election security, experts say, crystallize the "stark illustration" between the two men in their approaches to national security.
"The president seems to see foreign interference and election security issues as an opportunity to advance his political agenda," said John Cohen, an ABC News contributor and former Department of Homeland Security undersecretary for intelligence who worked in both the Bush and Obama administrations. "Joe Biden tends to view them as issues that need to be solved to protect the security and safety of America…as a way to articulate his vision on security."
The significance of election security is perhaps uniquely paramount this cycle, Cohen said, because "this election is happening in an extraordinarily volatile time in our country" -- between the coronavirus pandemic and the unrest over racial inequality.
And Trump "seems to be fanning the flames," he added.