Manigault Newman was previously best known for her polarizing character on NBC's "The Apprentice".
The accusations are among a long list of scandalous claims contained in her new book, "Unhinged", set to come out August 14. The Associated Press purchased an early copy.
Later, inside his club, where he met with Bikers for Trump - who wore Make America Great Again hats and leather jackets - and other supporters, the president responded with a dramatic flourish to a question about whether he felt betrayed by the Manigault Newman - one of the first White House hires, who once enjoyed unfettered access to him in the Oval Office.
"I didn't want to believe it", she writes. While she doesn't provide direct evidence, she cites three unnamed sources who attest to the existence of the tape.
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In the book, Manigault Newman painted the president as scattered, self-absorbed, misogynistic and insecure.
Trump would turn to hateful phrases when discussing White House counsellor Kellyanne Conway's husband George Conway, who is half Filipino.
"It had finally sunk in that the person I'd thought I'd known so well for so long was actually a racist", she wrote, according to The Guardian. She says she had seen him acting inappropriately at numerous events he attended without his wife at his Mar-a-Lago estate in Florida, including birthday parties, fundraisers and golf tournaments.
The book also says that there are tapes of Trump saying these slurs, but was never able to get a hold of them. The White House says the book is "riddled with lies and false accusations".
As reports of accounts inside the book leaked out, White House Press Secretary Sarah Sanders slammed Manigault Newman as a "disgruntled former White House staffer" whose book is "riddled with lies and false accusations".