Trump has made several comments about going after his political opponents and retribution. Asked to clarify what he meant, Trump replied, "Well, I said my retribution is going to be success."

He also contended that he's done nothing wrong despite his numerous criminal and civil cases.

Biden fired back contending that Trump's claims that he has the right to seek retribution is wrong. He then went after Trump over his various scandals.

"The crimes you are still charged with and think of all the civil penalties you have, how many billions of dollars ... and civil penalties? For molesting a woman in public for doing a whole range of things ... having sex with a porn star ... while your wife was pregnant? What would you be talking about? You have the morals of an alley cat," Biden said.

After a stifled and stumbling opening statement, Biden has had some good replies to Trump when talking about policy -- especially on abortion and Jan. 6. But that will be little comfort for his campaign, which is fighting hard against the impression that Biden is too old to hold office. In a 538/Ipsos poll of Americans released today, 40% of Americans said they rated Biden's mental fitness to be president as "terrible," compared to 35% for Trump -- and fewer Americans rate him positively for his physical fitness to lead, too.

538's election forecasting model currently gives Biden and Trump each a 50-in-100 chance of winning the election. Biden's initial impression could prevent him from making sizable gains over those numbers, despite the rest of his performance.

-538's G. Elliott Morris

When asked point-blank whether he believes the millions of Americans who are likely to vote for Trump will be voting against American democracy -- in reference to the president's past comment that Trump and his MAGA Republicans are "determined to destroy American democracy," Biden responded, "Yes, the more they know about what he's done."

Biden went on to reference Trump's comment that there were "very fine people on both sides" of clashes at a white supremacist rally in Charlottesville, Virginia, in 2017.

"This guy has no sense of American democracy," Biden said.

Trump called his accusation false.

False. During an exchange on the attack on the Capitol on Jan. 6, Trump pivoted to the quick action he says he took to stop rioting in Minneapolis after George Floyd was killed in the custody of a Minneapolis police officer.

"If I didn’t bring in the National Guard, that city would have been destroyed," Trump claimed.

It was Gov. Tim Walz who activated 500 Minnesota National Guardsmen to provide support to local law enforcement in Minneapolis.

-Analysis by Aaron Sharockman, PolitiFact

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