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

After tonight's debate, there will be a rush to anoint a "winner" and a "loser," but the only way we can really do that is once we have data on how the debate will actually affect people’s votes. To that end, 538 partnered with Ipsos to poll the same group of likely voters both before and after the debate to see how their attitudes change. Here are some of the key findings from our pre-debate poll, which was conducted using Ipsos' KnowledgePanel.

First, we asked respondents to rate how well they thought each candidate would perform in the debate tonight on a five-point scale. On average, Trump got a score of 2.96 out of 5, and Biden got a score of 2.58 out of 5. In other words, expectations are significantly lower for Biden tonight, which could end up helping him -- even a so-so performance from Biden would exceed most people's expectations.

It looks like the reason people have such low expectations for Biden is his advanced age. We also asked respondents to grade each candidate's physical, mental and emotional fitness on a five-point scale. On average, Biden got just a 2.3 out of 5 on physical fitness and a 2.4 out of 5 on mental fitness. Trump bested him on both of those measures, but Trump got only a 2.6 out of 5 on emotional fitness, which was lower than Biden's score.

We also asked voters what issues would have the most impact on their vote. Fifty percent ranked inflation or increasing costs as one of their top three issues, while 37% included immigration. Voters also said Trump would do a better job handling those issues than Biden, so it will be especially important for the president to show strength on these issues tonight.

Finally, we asked voters which candidates they were considering supporting. Heading into the debate, 44.8% of voters are at least considering voting for Trump, 44.5% are at least considering voting for Biden and 18.5% are at least considering voting for independent candidate Robert F. Kennedy Jr., who failed to qualify for the debate. (Respondents could say they were considering multiple candidates, which is why these numbers add up to more than 100%.) We'll ask voters the same question after the debate to see whether these numbers shift.

-538's Nathaniel Rakich