Sen. Heidi Heitkamp, D-N.D., apologized Tuesday for a campaign ad billed as an "open letter" to her challenger, Republican Rep. Kevin Cramer, that identified women as sexual assault survivors without their permission.

"This is a huge mistake that anyone was included on this letter that should not have been included," Heitkamp, who faces a tough re-election fight, told North Dakota radio host Rob Port Tuesday afternoon.

The letter, published in a North Dakota paper, was created in response to Cramer's recent comments that the #MeToo movement — which has helped to publicize and protest sexual assault and harassment against women — is a movement toward "victimization."

But some women who were listed as having signed the letter did not give permission for their names to be revealed, or were not survivors, her campaign acknowledged in a statement Tuesday morning.

"We recently discovered that several of the women's names who were provided to us did not authorize their names to be shared or were not survivors of abuse," Heitkamp said. "I deeply regret this mistake and we are in the process of issuing a retraction, personally apologizing to each of the people impacted by this and taking the necessary steps to ensure this never happens again."

The North Dakota Republican Party called it "another example of Heidi Heitkamp exploiting whoever she can for political gain."

"With a campaign built on lies, misinformation and manufactured controversy, it’s no wonder Heitkamp is the most vulnerable senator in the country," the state party's communications director, Jake Wilkins, said in a statement.

A woman whose name was listed on the ad, Lexi Zhorela, told The Associated Press that she was "furious."

"I have only shared my story with a couple of people in confidence," she said, according to the AP. "I didn't want it blasted for the world to see."

Heitkamp told Port, the radio host, that she hadn't seen the ad before publication. She also said she has been reaching out to people on social media, as well as looking for phone numbers so she can speak to those affected by the error.

"At the end of the day Rob, this is on me," Heitkamp said. "This has got my name on it. ... I’ve spent a great deal of my professional time working with victims, and the worst thing you could do is take away their privacy, and I feel like if in fact these women were listed who are victims, I did exactly that, and I need to personally apologize and say it was wrong."

Heitkamp, a former state attorney general, has spoken publicly about her own family's experience with sexual assault. After voting against the confirmation of Supreme Court Justice Brett Kavanaugh — and after Cramer made several dismissive comments about the #MeToo movement and the sexual misconduct allegations against Kavanaugh — Heitkamp told reporters that her own mother was sexually assaulted.

“I think it's wonderful that his wife has never had an experience, and good for her, and it’s wonderful his mom hasn’t,” she told The New York Times this month. “My mom did. And I think it affected my mom her whole life. And it didn’t make her less strong.”

Heitkamp, by far the Senate's most vulnerable Democratic incumbent in a red state, said Tuesday on the radio that she's "had better days."

A Fox News poll of likely voters in North Dakota released on Oct. 3 found Heitkamp trailing Cramer by 12 points. President Donald Trump won North Dakota by more than 30 points in 2016.