Russia's ongoing brazen efforts to interfere in the presidential election -- as they did four years ago -- remains a principal concern among national security officials, along with other adversaries including China. But unlike in 2016, the rapid and sweeping embrace of vote-by-mail in response to the coronavirus pandemic injects new uncertainty about election security, as millions prepare to vote in an alternative way.

Fueling those anxieties over mail voting, which is underway in several states, is President Trump’s unsubstantiated allegations of rampant fraud, even as dozens of state election officials tell ABC News that they have confidence in the system. His campaign against vote-by-mail throughout the cycle exceeds similar efforts he made in 2016, when he argued, without evidence, that voter fraud was the only reason Hillary Clinton won the popular vote. His own voting integrity commission found no evidence of that.

The integrity of this year's matchup between Trump and former Vice President Joe Biden has taken on an outsized role in the increasingly competitive political environment, as attempts to undermine confidence in the democratic process mount both from outside the country's borders and from inside the Oval Office.

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

-ABC News' Kendall Karson and Quinn Scanlan

A tale of two campaigns is emerging ahead of the first face-to-face meeting in the general election between President Donald Trump and former Vice President Joe Biden Tuesday night on the presidential debate stage.

While President Trump has hit the campaign trail hard, holding eight events in the last week, Biden has taken a decidedly different approach, keeping a light schedule in order to prepare for their upcoming meeting.

The former vice president has hunkered down for debate preparations, which are being overseen by Democratic debate guru and Biden's former chief of staff, Ron Klain, according to sources familiar with the preparation.

The preparations also includes Bob Bauer, a senior Biden adviser and former White House general counsel, stepping into the role of Donald Trump to help the former vice president get ready for Tuesday according to a source familiar with the process.

"Joe Biden is very big on preparing," said Patti Solis Doyle, a Democratic strategist and former adviser to Biden, who took part in his 2008 vice presidential debate preparations.

"He understands that this is an opportunity to really speak directly to the American people. And so he wants to make sure he's ready and prepared," she added, noting his fondness for briefing books and discussion of strategy.

Biden aides expect the former vice president to focus on making the case for his own presidency in his time on the stage, with Biden expected to contrast his vision with Trump's record -- particularly his response to the coronavirus pandemic.

More on Biden's debate prep can be found here

-ABC News' Molly Nagle

When President Donald Trump and former Vice President Joe Biden share a debate stage for the first time Tuesday night, a key audience for both candidates will be suburban women voters in a handful of critical election battlegrounds.

ABC News Live recently met with five women voters from diverse backgrounds and political views in the suburbs around Minneapolis to discuss the 2020 campaign and issues important to mothers, young female professionals and retirees across the state.

They described anxiety over a year of cascading crises for their families, from the pandemic and recession, to protests for racial justice, a spike in violent crime and looting and a sudden vacancy on the U.S. Supreme Court.

WATCH: Meet suburban women voters in the Twin Cities tonight at 7pm ET and 9pm ET on ABC News Live Prime.

-ABC News' Devin Dwyer

Leading up to Tuesday's debate, ABC News has broken down where each candidate stands on some of the key issues.

Throughout the day, we will be profiling each individual issue here.

Here is ABC News' coverage on some of the key issues:

Election security and integrity

Racial justice

Health

Economy

Climate change and the environment

President Donald Trump is set to enter the first presidential debate on Tuesday down in the polls, but coming off the high of nominating his third Supreme Court justice. Some top advisers expect a dominant performance by the president, despite the opposing narratives of attacks on Joe Biden's mental state and the Trump campaign's work to raise expectations for the Democrat ahead of the showdown.

With days to go the debate, Trump has carried out conflicting and confounding strategies both repeatedly targeting Biden's mental acuity, claiming at a recent rally he doesn't know he's alive, while also making passing attempts to boost the former vice president as a seasoned debater who could outshine him.

"This guy doesn't have a clue. He doesn't know where the hell he is," Trump told his supporters in Pittsburgh a week to the day before the first debate. "This guy doesn't know he's alive."

At a campaign rally Saturday night in Pennsylvania, the president both attacked Biden as "dumb" before immediately lauding him as an experienced debater. "He’s a dumb guy. Always known as a dumb guy. But we look forward to seeing him in the debate. He's got a lot more experience. He's got 47 years. I’ve got 3 1/2 years. So we'll see. But he's got 47 years of experience."

Biden has brushed off Trump's onslaught of attacks on his mental fitness. "Watch me, Mr. President. Watch me. Look at us both, what we say, what we do, what we control, what we know, what kind of shape we're in," the former vice president said when asked by ABC News about the president's attacks on his cognitive ability.

While some advisers tell ABC News they're nervous as the president enters his first debate since his encounters with Hillary Clinton four years ago -- with Biden having participated in nearly a dozen debates leading up to clinching the Democratic nomination -- others are anticipating a knockout performance by the president and expect the debate to help highlight the former vice president's gaffes, sources familiar said.

-ABC News' Will Steakin, Katherine Faulders and John Santucci