As Donald Trump hovered behind Hillary Clinton on stage, her "skin crawled".

Hillary Clinton says Donald Trump’s pacing, hovering demeanour onstage during an October 2016 presidential debate made her so uncomfortable her “skin crawled”.

In her upcoming book, Clinton writes Trump shadowed her so closely she had to resist shouting out, “back up you creep, get away from me”.

The Democratic presidential nominee recounts her struggle to keep composed during the pivotal face-off less than a month before the election.

Two days earlier, their bitter campaign was rocked by the release of footage in which Trump bragged aggressively about groping women.

During the town-hall style debate, the 6ft 3 (185cm) Trump repeatedly hovered over Clinton, who’s closer to 5ft 5 (165cm), as she responded to questions.

“This is not OK, I thought,” Clinton said in her audio narration of What Happened, set for release on September 12.

“It was the second presidential debate, and Donald Trump was looming behind me,” she describes.

“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.”

Donald Trump and Hillary Clinton during the town hall debate at Washington University on October 9. Image: Getty.

"It was one of those moments where you wish you could hit pause and ask everyone watching, 'Well, what would you do?' Do you stay calm, keep smiling and carry on as if he weren't repeatedly invading your space? 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."'

Clinton says she "kept her cool", aided by a lifetime of "dealing with difficult men trying to throw me off".

"Maybe I have over learned the lesson of staying calm, biting my tongue, digging my fingernails into a clenched fist, smiling all the while determined to present a composed face to the world," Clinton says.

Excerpts of Clinton reading from the book aired Wednesday on MSNBC.

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