In her new book about her time on the campaign trail, Hillary Clinton said her "skin crawled" when then-candidate Donald Trump loomed over her during a presidential debate.

"'This is not OK,'" she recalled thinking in What Happened, set to come out Sept. 12. "Two days before, the world heard him brag about groping women. Now, we were on a small stage and no matter where I walked, he followed me closely, staring at me, making faces. It was incredibly uncomfortable. He was literally breathing down my neck. My skin crawled."

The excerpt comes from an audio excerpt of the book, which was released Wednesday by MSNBC.

Trump's lurking during last year's second presidential debate in St. Louis on Oct. 9 turned heads at the time. The future president was awfully close to his Democratic challenger, who answered a question as he stood behind her.

Clinton said she gripped the microphone "extra hard" and was able to keep her cool because of "a lifetime of dealing with difficult men trying to throw me off."

But she did imagine the alternative, which she admitted would have made for better television.

"Do you stay calm, keep smiling and carry on as if he weren't repeatedly invading your space?" she asked herself. "Or do you turn, look him in the eye, and say loudly and clearly, 'Back up, you creep. Get away from me. I know you love to intimidate women, but you can't intimidate me, so back up.'"

The book, as she references in the audio clip, will feature parts of the campaign she remembers fondly and others she'd rather do over.

"If the Russians could hack my subconscious, they'd find a long list," she said.

She called the campaign, "exhilarating, joyful, humbling, infuriating and just plain baffling." She added she couldn't bear the idea of letting millions of people down.

"But I did," she conceded. "I couldn't get the job done and I'll have to live with that for the rest of my life."

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