The candidates were asked what they could do to help Americans battling the opioid crisis in the U.S.

After being prompted to respond to the question, Trump said that during his term as president they had "great equipment" and drug-detecting dogs to find illicit drugs at the border.

Biden said the U.S. needs machinery that can detect fentanyl and more agents at the border, and added that the U.S. is coming down "very hard" on countries in Asia that are producing fentanyl.

Moderator Dana Bash asked Biden, 81, and Trump, 78, to address voter concerns about their age.

"First of all, I spent half my career being criticized for being the youngest person in politics," Biden said. "I was the second youngest person ever elected to the United States Senate. And now I'm the oldest. This guy's three years younger and a lot less competent."

Biden said voters should look at his record and what his administration's accomplished.

Trump pointed to the "cognitive test" he's taken. In 2018, he took the Montreal Cognitive Assessment, a test designed to detect early signs of memory loss and other mild cognitive impairment.

"He took none," Trump said of Biden. "I'd like to see him take one, just one, a really easy one. Like go through the first five questions, he couldn't do it."

Biden had a poor start to this debate, and if he ends up being viewed as the "loser" tonight, he'd be just the latest incumbent president to go through this. Incumbent presidents like Ronald Reagan in 1984, George W. Bush in 2004, Barack Obama in 2012 and Trump in 2020 all lost 2 to 3 points in the polls after their first debate.

Reagan, the last president to face real questions about his age -- he was 73 in 1984 -- had a meandering performance that prompted questions about whether he had gotten too old for the job. Reagan shot back in the second debate against Democratic nominee Walter Mondale, who was in his mid-50s, with the famous line: "I will not make age an issue of this campaign. I am not going to exploit, for political purposes, my opponent's youth and inexperience." Reagan, however, also had a roaring economy, which helped him win a landslide reelection victory.

-Geoffrey Skelley, 538

An hour in and here is the Biden campaign spin on what so far has been a rocky performance by the president.

Biden has a cold (but tested negative for COVID-19), the campaign confirms. They haven't offered any explanation for why they didn't disclose this sooner.

Overall, the campaign says they are feeling fine. They agree it was a slow start but say he's found his groove.

-ABC News' Mary Bruce and Molly Nagle

Heading into tonight's event, Biden and Trump are locked in a close contest. 538's presidential election forecast rates the race as a dead heat, with both Biden and Trump having about a 1 in 2 shot of winning the election. This falls in line with our national polling average, which has the two candidates just about tied at 41%, with Kennedy polling at 9%.

But as readers know, U.S. presidential elections aren't decided by the national popular vote. Instead, they're decided by the Electoral College, where the results in individual states determine who wins each state's electoral votes, with a majority of 270 out of 538 in total necessary to claim victory. Currently, the forecast shows an extremely tight race in each of the most pivotal states.

For instance, Biden's path to victory may rest primarily on winning the Frost Belt battlegrounds of Michigan, Pennsylvania and Wisconsin that he carried in 2020. While the forecast shows him with small leads in each of those states, Biden's advantage is well inside the range of potential outcomes, meaning Trump could easily carry them once we get to November. Meanwhile, Trump holds a narrow edge in the Sun Belt swing states of Arizona, Georgia, Nevada and North Carolina, but similarly his leads are far from secure.

Still, with nearly four months to go until Election Day, there's a great deal of uncertainty around where public opinion will stand when voters cast their ballots. 538's forecast utilizes a combination of polling information and campaign fundamentals, such as the economic situation and each state's partisan leanings. Together, those indicators show a toss-up race that could go in either direction.

Now, tonight's debate could move the contest: Historically, the margin in presidential races has shifted by an average of about 2 points in the two weeks following the first general election debate in cycles dating back to 1976. In what could be bad news for Biden, that movement has more often than not gone against the incumbent president or party. Yet we're still sailing in uncharted waters. After all, this election involves both a sitting and former president, an unprecedented matchup in modern times. The debate is history-making, too: It's nearly three months earlier than any past general election face-off.

-538's Geoffrey Skelley