Former Vice President Joe Biden has told Oprah that he regrets he is not president while admitting he knew Hillary Clinton would not win the 2016 election.

"I have a regret that I am not president," Biden candidly told the talk show star. "Because I I think there's so much opportunity. I think America's so incredibly well-positioned."

Biden was speaking to Oprah as part of his book tour.

The title of the new book, Promise Me, Dad, comes from his late son Beau Biden, asking that he be okay as he faced the cancer that eventually killed him.

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He told Oprah that the grief of losing his son prevented him from entering the presidential race, which Donald Trump eventually won.

"Oprah, no woman or man should announce they are running for president unless they can answer two questions: One, do they truly believe they are the most qualified person for that moment. I believed I was," Biden said.

It was the second question that stopped him.

"But was I prepared to be able to give my whole heart, my whole soul and all my attention to the endeavour?" he said. "And I knew I wasn't."

Biden repeated that he knew, a month out, that Hillary Clinton's candidacy was doomed.

"A month out, I came back and said, we're going to lose this election," Biden said.

He also revealed that George Clooney had offered to work on his election campaign.

Clooney told Biden he was willing to provide
Clooney told Biden he was willing to provide "any and all assistance" to get him on the ballot. Photo / AP

Clooney told the former vice president he was willing to provide "any and all assistance" to get him on the ballot.

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The Hollywood actor hosted a lavish fundraiser for Clinton at his home last year and was presumed to be a hardcore Clinton fan.

"An executive in the entertainment industry had insisted that I had more support in the Hollywood community than Hillary," Biden writes in the book.

"He said I could raise money without a problem. George Clooney got in touch with Steve Ricchetti soon after that."