Amid questioning about democracy and the Trump mob attack on the U.S. Capitol, Biden took a shot at Trump's historic conviction in the hush money case.

"The only convicted felon is the guy standing on the stage with me now," Biden said.

Trump then quickly brought up Hunter Biden's recent conviction on federal gun charges.

Trump largely avoiding taking on a question about what he would say to Americans who believe he violated his oath on Jan. 6, 2021, and are worried he would do it again.

Instead, he pivoted to the border and to criticizing Biden's record.

Moderator Jake Tapper pressed him again, saying there was 80 seconds remaining to answer the question. Trump went on to attack former House Speaker Nancy Pelosi.<

In a heated moment in response to Trump's claim that the president "doesn't care about our veterans," Biden referenced a 2020 Atlantic article in which Trump was reported to have called Americans killed as "suckers" while on a trip to visit the Aisne-Marne American Cemetery in France in 2018.

On the subsequently canceled visit, the Atlantic reported that Trump told senior staff members, "Why should I go to that cemetery? It’s filled with losers."

Referring to his late son Beau during the debate, Biden said to Trump, "My son was not a loser, was not a sucker. You're the sucker, you're the loser."

Trump responded by saying it was in a "third-rate magazine" and the source had "made it up."

During the debate, Trump claimed that Democrats "support abortion up to and even beyond the ninth month," and contended that some Democratic states have had such a stance.

Democrats have never advocated for killing a baby after it is born as this would be murder, which is illegal in every state. Nine states have no abortion restrictions in place; however, it should be noted that late-term abortions are exceedingly rare, representing 1%, according to KFF.

As Biden and Trump prepare to square off in their first political debate in four years, historians and experts contend the matchup may have a small but crucial impact on the election.

Aaron Kall, director of debate for the University of Michigan's Debate Program, told ABC News the majority of those who tune in are likely already locked into a preferred candidate.

"Nothing that occurs during the 90-minute debate is going to change or influence who they're going to vote for," he said.
However, Kall and other experts ABC News spoke with said there is still a smaller group of undecided voters who do tune in and can be swayed by the performances.

With the last two presidential elections decided by just tens of thousands of votes in a few states -- many cast by independent voters -- candidates' debate strategies have become laser-focused on courting that group, according to Julien Labarre, administrator of the University of California Santa Barbara's Center of Information Technology & Society.

"What we see is people who were not thinking of going to vote being turned into voters," he told ABC News. "Spurring people into participation, we do see that kind of effect."

-ABC News' Ivan Pereira