TRUMP'S STATEMENT: "Once you became vice president, (Hunter Biden) made a fortune in Ukraine and China and Moscow and various other places. He made a fortune, and he didn't have a job."

FACT CHECK: While the exact sum of Hunter Biden's foreign business ventures is unclear, ethics experts have broadly taken issue with them -- particularly those in places like Ukraine and China, at a time when his father was fronting U.S. policy there.

Robert Weissman, the president of progressive watchdog group Public Citizen, told ABC News last year, "at absolute minimum there's a huge appearance of conflict … (Joe Biden) should have encouraged his son to not take these positions."

Still, Weissman called Hunter Biden's business activity "orders of magnitude" less troubling than how the Trump family has operated its businesses during the Trump administration.

In an interview with ABC News last year, Hunter Biden said he exercised "poor judgment," but maintained that he made no "ethical lapses."

"I gave a hook to some very unethical people to act in illegal ways to try to do some harm to my father. That's where I made the mistake," he said. "So I take full responsibility for that. Did I do anything improper? No, not in any way. Not in any way whatsoever."

"Did I make a mistake? Well, maybe in the grand scheme of things, yeah," he continued. "But did I make a mistake based upon some ethical lapse? Absolutely not."

In their 87-page report released last week, Senate Republicans concluded that Hunter Biden's overseas endeavors were "awkward" and at times "problematic," but stopped short of identifying any criminal activity on Hunter Biden's part or any wrongdoing on Joe Biden's part.

--ABC News' Lucien Bruggeman

The final question to both candidates was how will they reassure the American people that the next president will be the legitimate winner of this election.

Biden answered first by urging Americans to make a voting plan before they get to the polls, before assuring Americans that their mail-in votes would be counted.

"His own homeland security director as well as the FBI director says there is no evidence at all that mail-in ballots are a source of being manipulated and cheating. They said that the fact is that there are going to be millions of people because of COVID that are going to be voting by mail in ballots -- like he does by the way, he sits behind the Resolute desk and sends his ballot to Florida."

"Number two, we're going to make sure that those people who want to vote in person, are able to vote," Biden added.

Trump struck a different note warning of "fraud like you've never seen."

"As far as the ballots are concerned, it's a disaster," Trump said. "They're sending millions of ballots all over the country."

"I think we're going to do well because people are really happy with the job we've done. But you know what, we won't know -- we might not know for months, because these ballots are going to be all over," Trump added.

Elections officials in dozens of states have stood firm in expressing confidence to ABC News that they'll be able to effectively carry out expanded mail-in voting in November, despite Trump's repeated claims of rampant voter fraud.

BIDEN'S CLAIM: Biden says 100 million Americans have pre-existing conditions, Trump says he's wrong

FACT CHECK: 100 million or more people have pre-existing conditions in the U.S.

Biden said during the debate that "100 million people" have pre-existing conditions, while Trump said that number was "totally wrong."

"There aren't 100 million people with pre-existing conditions," Trump said. The two candidates were debating about the path forward for health care in America.

Biden, who was arguing that Trump and Republicans would kick people with pre-existing conditions off of health care if they rescinded the Affordable Care Act, is correct that somewhere around 100 million Americans have pre-existing conditions, according to studies.

One study by the Health and Human Services Department in 2017 found that a large percentage of non-elderly Americans have pre-existing health conditions, ranging from 61 million to 133 million.

The study found that "at least 23 percent of Americans (61 million people) using a narrow definition based on eligibility criteria for pre-ACA state high-risk pools, or as many as 51 percent (133 million people) using a broader definition closer to the underwriting criteria used by insurers prior to the ACA" could have been denied health care or offered it at a high price before the Affordable Care Act passed because of pre-existing conditions. The study was published under the Obama administration.

There is a difference, however, between the absolute number of people with pre-existing conditions and the number of people who would be denied coverage under the Affordable Care Act. Experts point out that 54 million non-elderly Americans would've been denied insurance before the ACA, but confirmed that 100 million or more Americans have pre-existing conditions.

"54 million non-elderly adults have a pre-existing condition that would have led to a denial of insurance before the ACA. 100 million or more have a pre-existing condition that would have led to higher premiums or limited benefits," tweeted Larry Levitt, executive vice president for health policy at the Kaiser Family Foundation, referring to a study by the organization.

When asked by Wallace if he believes that human pollution and greenhouse gas emission contributes to global warming, Trump responded that, “I think a lot of things do, but I think to an extent, yes.”

“I want crystal clean water and air,” Trump said. “I want beautiful clean air.”

But Trump has previously publicly said he isn't sure how much humans affect the climate. He also pulled the United States out of the Paris Climate Agreement.

Biden, on the other hand, has said he would recommit to the Paris Climate Agreement. He also said if he was elected in November, he would aim to have the U.S. reach zero emissions by 2050.

Wallace went on to press Biden on the cost of his climate change plan noting it includes upgrading 4 million buildings and weatherizing 2 million homes over four years. Biden said the plan will create millions of good jobs and drew the distinction that the Green New Deal is not his plan.

"We're going to be in a position where we can create, hard, good jobs by making sure the environment is clean, and we all are in better shape," Biden said.

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