Former US First Lady Jill Biden says she thought her husband, President Joe Biden, was having a stroke during his infamously faltering presidential debate in 2024.

"I was frightened, because I had never ever seen Joe like that before or since. Never," Jill Biden told CBS News, the BBC's US partner.

"I don't know what happened," she said. "As I watched it, I thought, 'Oh, my God, he's having a stroke.' And it scared me to death."

President Biden faced intense pressure from fellow Democrats to step down as their candidate after a poor debate performance against then-Republican candidate Donald Trump.

Biden ultimately dropped out of the race and endorsed then-Vice President Kamala Harris.

The former first lady's interview with CBS News Sunday Morning's Rita Braver will air on Sunday.

Jill Biden has stood beside her husband throughout his decades-long career, from his time as a Delaware senator to his years as president.

She was considered to be one of his most influential advisers during his presidency, and among those who ultimately encouraged him to drop out of the 2024 race.

Following the June 2024 debate, months ahead of the presidential election, Democrats expressed unease at the president's shaky debate performance.

Biden's campaign at the time insisted the president would not step down as the Democratic candidate and that he would debate Donald Trump again.

During the debate in which both men sought a second term, Biden and Trump sparred on major topics including immigration, the economy and abortion rights.

Biden had a raspy voice, which his team said was due to illness, and a moment when he seemed to lose his train of thought.

At a post-debate rally in Atlanta, Jill Biden introduced her husband on stage and praised his performance against Trump.

"Joe, you did such a great job. You answered every question. You knew all the facts," she told the crowd.

But elsewhere, Democratic leaders and donors expressed concerns about the then-president's performance. Kamala Harris, who rarely criticised Biden publicly, even admitted it was a "slow start" to the debate.

Voters' concerns about Biden's health, mental acuity and age - 81 at the time - already weighed heavily on the debate.

Analysts piled on the president across most major US media outlets, with concerns raised about Biden's ability to do the job of president as well as his chances in the November 2024 election.

It was thought unlikely that Biden, the incumbent president, would be replaced as the Democratic Party's nominee just a few months before the election. The difficult process of choosing another nominee was seen as having the potential to derail the party's White House bid.

But his debate performance - along with gaffes during a Nato summit in the following weeks and a frail demeanour after a Covid diagnosis - ultimately led to him ending his candidacy.

Harris assumed the Democratic nomination about three months before the election and ultimately lost her bid to Trump.

In the aftermath of her election loss, Harris criticised her former boss, calling Biden's decision to seek a second term "recklessness".

"'It's Joe and Jill's decision.' We all said that, like a mantra, as if we'd all been hypnotised," Harris wrote in her memoir. "Was it grace, or was it recklessness? In retrospect, I think it was recklessness."