Donald Trump Jr., who is a part of his father's re-election campaign, pointed to some of his father's accomplishments -- a great economy before COVID-19 and getting closer to achieving peace in the Middle East -- before moving onto his father's opponent.

When pressed by ABC News Chief Anchor George Stephanopoulos about his father's tax record and debt, which was first reported by the New York Times, Trump Jr. went on the attack.

"If people have a problem with the tax code, talk to Joe Biden," Trump Jr. said. "He's the guy that's been legislating these things or 47 years."

Outside the debate site in Cleveland on Tuesday, protesters were seen wearing costumes -- appearing to criticize Trump's Supreme Court nominee Amy Coney Barrett -- and holding signs in support of Black Lives Matter.

One photographer captured a Trump supporter in a red "Make America Great Again" hat and a counter protester in a shirt reading "racism is a public health issue" arguing ahead of the debate.

Democratic vice presidential nominee Sen. Kamala Harris will be watching the debate from Howard University with her husband Doug Emhoff and members of her staff, according to an aide. Harris earned bachelor's degrees in political science and economics at the university in Washington, D.C.

--ABC News’ Averi Harper

ABC News Chief Global Affairs Correspondent Raddatz said Trump is combative when he debates, saying "dealing with Donald Trump is more like being a lion tamer than a debate moderator. You really have to keep him on time. He'll go after the moderator."

Biden, Raddatz said, often uses humor when he debates.

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