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LAS VEGAS — Elizabeth Warren unleashed a stinging assault on Mike Bloomberg at the Democratic debate Wednesday night over his past comments about women and his refusal to release women who worked for him from nondisclosure agreements.

"I hope you heard what his defense was: 'I've been nice to some women,'" Warren said in mocking her rival. "That just doesn't cut it. The mayor has to stand on his record. What we need to know is exactly what's lurking out there."

"He has gotten some number of women — dozens, who knows — to sign nondisclosure agreements, both for sexual harassment and for gender discrimination in the workplace," Warren continued, referring to Bloomberg's namesake financial services company. "So, Mr. Mayor, are you willing to release all of those women from those nondisclosure agreements so we can hear their side of the story?"

Multiple women have told ABC News that they were interested in publicly sharing their stories about working for Bloomberg, the former mayor of New York, but were bound by confidentiality agreements and feared retaliation if they were to speak out.

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Warren, a senator from Massachusetts, continued her attack, pressing Bloomberg to tell the crowd at the NBC/MSNBC debate how many nondisclosure agreements there were — but he refused.

"They sign the agreements, and that's what we're going to live with," Bloomberg said of the women.

"We are not going to end these agreements, because they were made consensually," he continued, adding that the agreements were made between "two parties that wanted to keep it quiet."

Warren pressed: "I'm sorry, no. The question is are the women bound by being muzzled by you?"

Bloomberg has come under fire for a booklet that was circulated in the 1990s documenting sexist and offensive comments he was alleged to have made to women who worked for him. In one of the most high-profile lawsuits brought against Bloomberg, one woman accused him of telling her to "kill it" when he learned that she was pregnant. Bloomberg reached a confidentiality agreement with the woman.

Warren opened up the debate by referring to the allegations in the booklet, saying: "I'd like to talk about who we're running against, a billionaire who calls women 'fat broads' and 'horse-faced lesbians.' And no, I'm not talking about Donald Trump. I'm talking about Mayor Bloomberg."

Bloomberg has denied many of the allegations.

Former Vice President Joe Biden jumped in and also pressed Bloomberg to negate the nondisclosure agreements so the women could speak freely.

"All the mayor has to do is say, 'You are released from the nondisclosure agreements,'" Biden said.

Bloomberg said: "We have very few nondisclosure agreements. None of them accuse me of doing anything other than maybe they didn't like a joke I told."