By Cooper Allen and Catalina Camia

USA TODAY

Updated 6:08 p.m. ET

Newt Gingrich sought an "open marriage," his former wife, Marianne, told ABC News in an interview airing tonight.

Marianne Gingrich told ABC's Nightline that the former House speaker asked her if he could remain married to her while also seeing another woman, Callista Bisek, his current wife.

Marianne Gingrich added that she doesn't believe Gingrich has the moral character to be president.

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Gingrich declined to comment about the specifics of the interview -- airing two days before the South Carolina primary -- when talking to reporters in Beaufort, S.C. He said the interview is "tawdry and inappropriate."

Gingrich's three marriages have been an issue for some social conservatives in as he seeks the GOP presidential nomination. He has acknowledged he has made mistakes in his life and has sought God's forgiveness .

Before excerpts were released, Gingrich dismissed the interview as an old story that isn't particularly newsworthy.

He told NBC's Today show this morning: "I'm not going to say anything negative about Marianne" as he objected into ABC News "intruding into family things that are ... more than a decade old."

Marianne Gingrich described for ABC News her "shock" over the details of Gingrich's affair, which coincided with him leading public criticism of President Clinton for his relationship with Monica Lewinsky.

She said she discovered that the affair had occurred "in my bedroom in our apartment in Washington."

"He always called me at night," she said, "and always ended with 'I love you.' Well, she was listening."

In an interview with The Washington Post, Gingrich said she learned of her husband's divorce request while having dinner with her mother on her mother's 84th birthday. The next day, Gingrich says, the GOP politician gave a speech on values.

"How could he ask me for a divorce on Monday and within 48 hours give a speech on family values and talk about how people treat people?" she said to The Post.

The Post reports that a Gingrich campaign spokesman did not answer their requests for a comment.

Newt Gingrich said his daughters, Kathy and Jackie, urged ABC News not to air the interview, which comes as he is chipping into Mitt Romney's lead in South Carolina polls.

ABC News posted a story on the interview and a video clip on its website, ahead of a GOP presidential debate tonight in Charleston. The interview with Marianne Gingrich is set to air during tonight's Nightline program at 11:35 p.m. ET.

ABC News spokesman Jeffrey Schneider downplayed reports of internal controversy over airing the interview.

"If there was a 'civil war,' as the New York Times said, it was a pretty short and bloodless one," he said. He added that it was always important to be "thoughtful" about airing stories close to an election.

By running the interview today, he said, there is time for the campaign to respond if needed.

Gingrich said on NBC that the American people will in the end make decisions about his character.

"People will have to judge me," Gingrich said. "But 16- and 20-year-old stories, you know, we have real stories this week on the failure of the Obama administration."

(Contributing: Martha T. Moore; Associated Press)