Moderator Chris Wallace has repeatedly tried to keep Trump from interrupting Biden.

"It's hard to get any word in with this clown," Biden said, "excuse me, this person."

Amid bombshell reporting in the New York Times on the past two decades of Trump's taxes, Wallace put the first question to the president.

"Is it true that you paid $750 in federal income taxes?" Wallace asked.

"I paid millions of dollars in taxes," Trump responded. "Millions of dollars of income tax."

But the president never fully answered if that was federal income tax.

Trump paid just $750 in income taxes in 2016, the year he ran for president, and in 2017, his first year in the White House, according to the Times report.

Biden, meanwhile, paid nearly $288,000 in federal income taxes last year, according to returns he released just hours before his Tuesday night debate, an effort to draw a sharp contrast with the revelations about Trump's tax returns.

Democrats said if the president is upwards of $400 million in debt as the New York Times has reported, he poses a national security threat to the country.

Wallace pressed on, asking "Mr. President, I'm asking you a question. Will you tell us how much you paid in federal income taxes in 2016 and 2017?

"Millions of dollars," Trump replied.

The president then blamed Biden for the tax codes since Biden has worked for so long in Washington.

"I'm going to eliminate the Trump tax cuts," Biden said.

"You are the worst president America has ever had," Biden continued.

"Let me just say, I've done more in 47 months than you've done in 47 years, Joe," Trump replied.

In a heated discussion on tax policy, Biden criticized Trump's trade policies with China which led to the first thinly veiled mention of Hunter Biden on stage.

"He talks about the art of the deal. China's perfected the art of the steal. We have a higher deficit with China now than we did before, we have the highest deficit -- trade deficit with Mexico," Biden said, before Trump interrupted.

"China ate your lunch, Joe. And no wonder your son goes in and he takes out what he takes out, billion of dollars," Trump said, before launching personal attacks on Biden's son, Hunter Biden, and his business dealings while his father was vice president. Hunter Biden has denied any wrongdoing.

"We want to talk about families and ethics? I don't want to do that. I mean his family, we can talk about all night," Biden responded, then said the American people don't want to hear about their families.

Asked about how they would deal with economic recovery, Trump said he would avoid further shutdowns, touted the current economic recovery and credited himself with bringing back college football.

"Our country is coming back incredibly well, setting records as it does it. We don't need somebody to come in and say let's shut it down," Trump said, though Biden said he would not impose a full shutdown as the president suggested.

Biden then took the opportunity to speak directly to American families on Trump's taxes and repeated his mantra that the U.S. "can't fix the economy without fixing COVID."

"The difference is millionaires and billionaires like him in the middle of the COVID crisis have done very well," Biden said. "You folks living in Scranton and Clayton and all the small towns and working-class towns in America: How well are you doing?"

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