Before Thursday evening, many Americans had expressed concerns about Joe Biden’s age and fitness for office. To say that this debate did not put those concerns to rest may be one of the greatest understatements of the year.

The president came into the debate with a low bar to clear, and he stumbled. He was flat. He was rambling. He was unclear.

About midway through the debate, the Biden campaign said that the president was battling a cold - an attempt to explain his raspy voice. That may be so, but it also sounded like an excuse.

For 90 minutes, more often than not, he was on the ropes. Particularly early in the evening, some of his answers were nonsensical. After losing his train of thought he ended one answer by saying, “We finally beat Medicare” – an odd reference to the government run healthcare programme for the elderly.

Mr Biden's own former communications director Kate Bedingfield was on CNN immediately after the debate, and she was clear: "There's no two ways about it, that was not a good debate for Joe Biden."

She said his biggest issue was to prove he had the energy and the stamina, and he didn't do that.

As the debate progressed, like a boxer on the ropes, Mr Biden started to take big swings against his opponent in an attempt to change the momentum. A few of those swings landed, provoking Donald Trump into angry responses

That the first few topics raised by CNN's moderators were on the top voter issues of economy and immigration - which polls show Americans trust Trump more on - only made the problem worse for the president, however.

“I really don’t know what he said at the end of that sentence, and I don’t think he did, either,” Trump said after another Biden response. That line may have summed up the night.

Trump largely offered a disciplined, nimble performance. He avoided the kind of interruptions and belligerence that undermined his first debate showing in 2020 and turned the discussion back to attacks on Mr Biden’s record whenever possible.

He repeatedly made assertions that weren't supported by facts as well as outright falsehoods, but Mr Biden largely was unable to corner him on them.

When the topic turned to abortion, for instance, the former president repeatedly shifted attention to what he said was Democratic extremism. He claimed, incorrectly, that Democrats support abortions after babies are born.

Abortion is an issue that has proven to be a weakness for Trump and Republicans in general since the overturning of Roe v Wade - which had protected the constitutional right to abortion - by the Supreme Court in 2022. But Mr Biden’s attacks in an area where he could have scored points fell flat.

“It’s been a terrible thing, what you’ve done,” the president said.

Shortly after the debate finished, Vice-President Kamala Harris acknowledged that the president had a “slow start” but said that he had finished strong. That is overly optimistic spin, but it’s true that Mr Biden steadied himself as the debate progressed.

In one memorable line, the president noted Trump's conviction on charges that stemmed from an alleged romantic liaison with adult film star Stormy Daniels, and said his opponent had “the morals of an alley cat”.

“I didn’t have sex with a porn star,” Trump snapped back.

Trump also appeared on the back foot when talking about his response to the 6 January attack on the US Capitol. He initially tried to turn a question about his responsibility for the riot into a condemnation of Mr Biden’s record, but this time the president wouldn’t let him off the hook.

“He encouraged those folks to go up to Capitol Hill. He sat there for three hours as his aides begged him to do something,” Mr Biden said. “He didn’t do a damn thing.”

The former president also repeatedly ducked and dodged when it came to whether he would accept the outcome of the 2024 election.

This was the earliest debate in modern US history, in part because the Biden team wanted it that way. One reason is they sought to shift focus to Trump earlier in the campaign season, hoping that American voters would be reminded of the chaotic nature of his presidency.

But more people will be talking about Mr Biden’s performance rather than Trump's after this debate.

Another reason the Biden team may have wanted an early debate is that it would give their candidate more time to recover from a weak performance. In the end, this may be what gives them comfort after Thursday night.

The Democrats have their party convention in August, when they will be able to offer a more scripted vision of a second Biden term for Americans. There is another debate in September, which - if it happens - will be fresher in American minds as they head to the polls in November.

But many Democrats may be wondering whether a second chance at Trump on the debate stage will turn out any different for their man. And some, at this point, may be thinking about how they might get a different presidential nominee.

The Biden campaign has almost two months to calm the waters. It would take an open revolt for Democrats to abandon their candidate – who comfortably won enough primary delegates to secure the party's nomination.

At least so far, no prominent Democratic officials have publicly broken ranks, even as some have reportedly raised alarms in anonymous conversations with journalists.

When asked by the BBC about the possibility of opening the convention to other candidates or replacing the president, Deputy Campaign Manager Quentin Fulks said they were “not going to dignify that with a response”.

“President Biden is going to be the Democratic nominee, and President Biden is going to win this election,” he added.

If Mr Biden’s campaign can rally his troops in the days ahead, the first assertion may be true. Trump himself has proven that politicians can face daunting adversity and grind through.

After this debate, however, many Democrats may have serious doubts about his prospects for November.