Former US vice president Kamala Harris said it was "recklessness" to let Joe Biden run for a second term as president, in an excerpt released on Wednesday from her upcoming memoir.

Harris, who replaced Biden as the 2024 Democratic presidential candidate but lost to Donald Trump, admitted that the then-81-year-old got "tired" and was prone to stumbles that showed his age.

The 60-year-old also accused Biden's team in the White House of failing to support her while she was his deputy, and at times of actively hindering her.

Harris said in the in the first extract from "107 Days", published by The Atlantic magazine, that she was a "loyal person" but asked whether during the "months of growing panic, should I have told Joe to consider not running?"

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

"The stakes were simply too high. This wasn't a choice that should have been left to an individual's ego, an individual's ambition. It should have been more than a personal decision."

Biden stunned the world by dropping out of the race in July 2024 after a disastrous debate with Trump sparked questions about his age and mental acuity.

Harris denied that there had been any conspiracy to hide Biden's condition but said it was clear there were issues with his age.

"On his worst day, he was more deeply knowledgeable, more capable of exercising judgment, and far more compassionate than Donald Trump on his best," she wrote.

"But at 81, Joe got tired. That's when his age showed in physical and verbal stumbles."

Harris also lashed out at White House staff whom she said failed to support her when she was vice president, saying that Biden's team did not want her to outshine her boss.

"When the stories were unfair or inaccurate, the president's inner circle seemed fine with it. Indeed, it seemed as if they decided I should be knocked down a little bit more," Harris wrote.

She added that she had "shouldered the blame" for Biden's border immigration policy, which Trump capitalised on in the election.

Harris also pointed to a March 2024 speech in which she criticised the humanitarian situation resulting from Israel's war in Gaza, breaking ranks with Biden's administration.

"It went viral, and the West Wing was displeased. I was castigated for, apparently, delivering it too well," Harris said.

"Their thinking was zero-sum: If she's shining, he's dimmed."

Harris also appeared hurt that Biden barely referred to her in his televised Oval Office address after dropping out of the race and anointing her as his successor as Democratic nominee.

"It was almost nine minutes into the 11-minute address before he mentioned me," she said.

Harris went on to lose comprehensively to Republican Trump after the shortest presidential campaign in modern US history, lasting just over three months – the 107 days in the title of her memoir. (AFP)