Pre-decided topics included the two candidates' records, the coronavirus response, the Supreme Court, the economy and voting issues -- though viewers only got a sense of each candidate's platform in brief moments, when they let each other answer.
"Fox News Sunday" anchor Chris Wallace, who had the trying task of keeping the squabbling under control, moderated the 90-minute debate from Case Western Reserve University in the Cleveland Clinic in Cleveland, Ohio.
With just over 30 days until Nov. 3, it's the first of three presidential debates of the cycle. But because mail-in voting has become a popular, safe choice during the pandemic, on Tuesday night, voting was underway as Trump and Biden sparred on stage.
According to Michael McDonald, a professor of political science at the University of Florida, over 1.1 million Americans have already received an absentee ballot in the mail.
Here are five key takeaways from the first of three presidential debates:
Squabbling leads to incoherence, leaving key questions unanswered for Americans
It took just 18 minutes for the debate to devolve into constant interruptions and halted sentences that made the candidates' answers incoherent, largely because of Trump's efforts to sidetrack and distract Biden, which often worked.
But Biden, though he got sidelined, didn't engage with the same level of personal attack and worked to keep his cool up against the president, who came in hot. Biden consistently looked at the moderator and the camera, talking to the viewers. Trump's body language was a sharp contrast; he was turned toward Biden throughout much of the debate, frequently pointing his finger at him.
Constantly, Trump interrupted Biden with one-sentence, off-subject jabs as the Democratic nominee tried to get through policy-focused responses regarding climate change, health care and protests over racial injustice.
Constantly, moderator Chris Wallace struggled to get the candidates to give complete answers on major issues facing Americans.
"Frankly, you've been doing more interrupting than he has," Wallace told Trump during the debate, struggling with the awkward task of rebuking the sitting president.
"The second subject is COVID-19, which is an awfully serious subject, so let's try to be serious about it," Wallace said, coming off a segment where Biden, losing his patience, asked Trump, "Will you just shut up, man?"
Again and again, the task of keeping the candidates on track was monumental. "Gentlemen! I hate to raise my voice," Wallace said about an hour into the debate, his patience wearing thin. "Why shouldn't I be different than the two of you?"
"The country would be better served if we allow both people to speak with fewer interruptions. I'm appealing to you to do that," he said, a pleading tone in his voice.
Supreme Court debate front and center through the lens of health care
"I will tell you very simply, we won the election. Elections have consequences," Trump said.
Should Coney Barrett's nomination be accepted by the Senate, conservatives will have a larger majority on the court. That possibility has ignited anger among some, who argue that the winner of the November election should choose a new appointee.
"We should wait and see what the outcome of this election is, because that's the only way the American people get to express their view is by who they elect as president and who they elect as vice president," Biden said.
Biden argued that top issues facing Americans are on the ballot by way of the court, which is soon deciding on the constitutionality of the Affordable Care Act.
"The justice -- and I have nothing -- I'm not opposed to the justice, she seems like a very fine person, but she's written before she went on the bench, which is her right, that she thinks that the Affordable Care Act is not constitutional," Biden said.
The Supreme Court is set to take up a case brought by the Trump administration to overturn the Affordable Care Act just days after the Nov. 3 election.
"There are 20 million people getting health care through Obamacare now that he wants to take it away. He won't ever look you in the eye and say that's what he wants to do, take it away," Biden said of Trump.
Trump's taxes have a brief moment in the spotlight
After a New York Times report which showed that Trump had paid only $750 in federal income taxes the year he took office, the Biden campaign has been attacking the president. On Tuesday, they advanced those attacks by releasing Biden's 2019 tax returns, which show the former vice president paid over $300,000 in taxes.
Asked onstage about the article, Trump maintained that he's paid more than the New York Times reported. "I paid millions of dollars in taxes. Millions of dollars of income tax," Trump said.
Still, Trump didn't specifically answer Wallace's question about how much he paid in federal income taxes, and instead just continued claiming that he's paid "millions."
Biden interjected, demanding that Trump release his tax returns as proof. The president said he will, "once they are ready," though he hasn't released his tax returns during his years as a candidate or during his years in office.
"I don't want to pay tax. Before I came here, I was a private developer, I was a private business people. Like every other private person, unless they're stupid, they go through the laws and that's what it is. He passed a tax bill that gave us all these privileges for depreciation, and for tax credits," Trump said.
Biden said his tax plan will prevent similar behavior from wealthy Americans in the future.
"He says he's smart because he can take advantage of the tax code. And he does take advantage of the tax code. That's why I'm going to eliminate the Trump tax cuts... I'm going to eliminate those tax cuts," Biden said.
Trump doesn't condemn white supremacy after he's asked -- and Biden misses an opportunity to dive in
Asked bluntly by Wallace if he would condemn white supremacists during Tuesday's debate, Trump, amid crosstalk and pivoting, refused to issue an outright denouncement.
His waffling comes as Americans nationwide have been in the streets protesting over the deaths of Black Americans at the hands of police. Some of those protests turned violent, like one in Kenosha, Wisconsin, where a young white teenager shot two protesters in the street.
Such protests added fuel to the fire over discussions of white supremacists, antifa and other extremist groups.
"But are you willing tonight to condemn white supremacists and militia groups... and to say that they need to stand down and not add to the violence in a number of these cities, as we saw in Kenosha and as we've seen in Portland?" Wallace asked.
"I'm willing to do anything, I want to see peace," Trump responded.
"Then do it, sir," Wallace said.
Trump didn't condemn the groups, and instead turned the question on Biden.
"Who would you like me to condemn? Who? Proud Boys, stand back and stand by," Trump said, referring to a far-right group, the Proud Boys, which recently rallied in Portland. "But I'll tell you what, somebody has got to do something about antifa and the left because this is not a right-wing problem, this is a left-wing problem."
Biden didn't take the opportunity to press Trump on the issue, instead telling him that he has "bad ideas" before Wallace moved onto another topic.
A simple question about voting yields two very different answers
The last question of the night was twofold: Will you condemn civil unrest as the country waits for results after Election Day, and will you pledge that you won't accept victory until the election has been officially verified?
Though in other years it would sound dramatic, this year, the question is a necessary one. Around the country, votes will likely take days or weeks to be counted because of the high number of mail-in ballots that are expected due to the pandemic. Though it'll change the timeline of when Americans are used to getting results, there is no evidence that it will change the validity of the election.
From Trump, the answer, essentially, was no -- though he didn't say it outright. From Biden, the answer was "yes" to both.
"I hope it's going to be a fair election. If it's a fair election I am 100% on board. But if I see tens of thousands of ballots being manipulated, I can't go along with that," Trump said.
Trump didn't give any reason to believe he thinks it will be a fair election, despite no evidence that there's likely to be fraud related to mail-in ballots. He called mail-in voting a "sham" and stoked fear, claiming that Americans might not know the results "for months."
Biden, on the other hand, called mail-in voting "honest." "No one has established at all that there is fraud related to mail-in ballots," he said.
He urged Americans to vote if they disagree with Trump, which he said was the antidote to Trump's argument.
"This is all about trying to dissuade people from voting because he's trying to scare people into thinking that it's not going to be legitimate. Show up and vote. You will determine the outcome of this election. Vote, vote, vote," Biden said.
And though there's no way for Biden to predict what Trump will do, he declared that both he and the president would accept the outcome of the election.
"The fact is I will accept it. And he will too. You know why? Because once the winner is declared after all the ballots are counted, all the votes are counted, that will be the end of it. That will be the end of it. And if it's me, fine. If it's not me, I will support the outcome," Biden said.
This report was featured in the Wednesday, Sept. 30, 2020, episode of “Start Here,” ABC News’ daily news podcast.
"Start Here" offers a straightforward look at the day's top stories in 20 minutes. Listen for free every weekday on Apple Podcasts, Google Podcasts, Spotify, the ABC News app or wherever you get your podcasts.