George Clooney has said it was a "mistake" for Kamala Harris to replace Joe Biden as the Democratic candidate to face Donald Trump in the US presidential election last November.

But the actor added that he had no regrets about writing an op-ed in the New York Times that July calling for Biden to quit the race.

In the piece, titled "I Love Joe Biden. But We Need a New Nominee", Clooney wrote that the ageing president had won many battles in his career "but the one battle he cannot win is the fight against time".

Clooney's comments come after the former president's son, Hunter Biden, lashed out at him for questioning his father's mental sharpness.

Less than a fortnight after Clooney's op-ed, Biden announced he would step aside for Harris.

In his interview with CBS, the actor said that he would write it again, adding: "We had a chance."

"I wanted there to be, as I wrote in the op-ed, a primary. Let's battle-test this quickly and get it up and going," he said.

But there was no Democratic primary, and Biden's vice president took the nomination, going on to lose against Trump.

"I think the mistake with it being Kamala is that she had to run against her own record. It's very hard to do if the point of running is to say, 'I'm not that person'. It's hard to do, and so she was given a very tough task," Clooney said.

"I think it was a mistake, quite honestly."

In the op-ed, the actor and prominent Democratic fundraiser wrote that it was "devastating to say it", but the Joe Biden he had met at a fundraising event three weeks earlier was not the Biden of 2010. "He wasn't even the Joe Biden of 2020," he added.

"He was the same man we all witnessed at the debate," Clooney said, in reference to Biden's disastrous TV debate against Trump weeks before, which fuelled new concerns about the 81-year-old's and fitness for office.

In an expletive-filled interview with the YouTube outlet Channel 5 with Andrew Callaghan, Hunter Biden accused Clooney of exaggerating the former president's frailty.

Asked why Clooney had intervened in the race, Hunter Biden responded with a succession of profanities about the actor.

"What do you have to do with… anything?" he said in a message directed at Clooney. "Why do I have to… listen to you?"

In an interview with the BBC last month, Harris said she might run again for the White House.

In her first UK interview, Harris said she would "possibly" be president one day and was confident there would be a woman in the White House in future.