Former Democratic presidential candidate Hillary Clinton had the perfect opportunity to call out her Republican rival Donald Trump's intimidatory stance in front of millions of Americans, but she didn't. And now she's wondering if that was the right decision.

Hillary Clinton: 'We need strong women to step up and speak out'

Clinton is about to release a much-anticipated book, titled What Happened, detailing her experiences in the 2016 campaign. In audio excerpts aired on MSNBC on Wednesday, she recalled the now-infamous second debate of the campaign, in St. Louis, on October 9, 2016.

The debate was staged just a few days after the release of the Access Hollywood tapes in which Trump was heard boasting about committing sexual assault. It looked like a watershed moment for the presidential campaign.

During the debate, Trump took an aggressive stance, following Clinton around the stage and "literally breathing down" her neck, as she recalled.

"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," Clinton said in the book.

The former secretary of state and first lady, who has taken on the cause of women's rights for much of her career, looks back at that moment as presenting her with a choice.

"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.'"

Clinton said she chose the first option, deciding that keeping her cool was the best way to deal with yet another "difficult" man who was trying to throw her off. To this day, though, she wonders if she should have gone for the second option and stood up to Trump's behavior on live television.

Newsweek subscription offers >

"It certainly would have been better TV," she quipped, adding, "Maybe I have overlearned 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."

The book is due for release in September, but already features at the top of the Amazon.com's best-selling list, thanks to preorders.

The book also deals with other key moments of the campaign that Clinton wishes she could either remember forever or go back to and do over. She said the writing process "wasn't easy" and acknowledged her defeat let down many Americans who wanted her to make history as the first female U.S. president.

"I knew that millions of people were counting on me, and I couldn't bear the idea of letting them down. But I did," she said. "I couldn't get the job done, and I'll have to live with that for the rest of my life."