Washington ? Dredging up a past that Newt Gingrich has worked hard to bury, the GOP presidential candidate’s second ex-wife says Gingrich asked for an “open marriage” in which he could have both a wife and a mistress.

In an interview with ABC News’ “Nightline” scheduled to air Thursday night, Marianne Gingrich said she refused to go along with the idea that she share her husband with Callista Bisek, who would later become his third wife.

The explosive interview was airing just two days before the presidential primary in South Carolina, a state with a strong Christian conservative bent, and as Gingrich tries to present himself as the strongest alternative to GOP front-runner Mitt Romney.

In excerpts of the interview released ahead the ABC broadcast, Marianne Gingrich said Gingrich conducted his affair with Callista “in my bedroom in our apartment in Washington.”

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

Marianne Gingrich, who was Gingrich’s second wife, said Gingrich told her “Callista doesn’t care what I do.”

“He was asking to have an open marriage and I refused,” she said. “That is not a marriage.”

She also said Gingrich moved to divorce her just months after she was diagnosed with multiple sclerosis.

“He also was advised by the doctor when I was sitting there that I was not to be under stress,” she said. “He knew.”

Gingrich, interviewed Thursday on NBC’s “Today” show, was asked about his ex-wife’s interview and declined to speculate on how it would affect his campaign.

He said he wouldn’t “say anything bad” about his ex-wife and that he preferred not to address his personal life in detailed fashion. He added that members of his family had written ABC to protest the airing of the interview, saying they complained about the network “intruding into family things that are more than a decade old.”

Marianne Gingrich has said that Gingrich proposed to her before the divorce from his first wife was final in 1981; they were married six months later. Her marriage to Gingrich ended in divorce in 2000, and Gingrich has acknowledged he’d already taken up with Bisek, a former congressional aide.

The House speaker who pilloried President Bill Clinton for his affair with Monica Lewinsky was himself having an affair at the time.

As plans to air the interview were disclosed, Gingrich’s campaign released a statement from his two daughters from his first marriage, Kathy Lubbers and Jackie Cushman, suggesting that Marianne Gingrich’s comments may be suspect given the emotional toll that divorce takes on everyone involved.

“Anyone who has had that experience understands it is a personal tragedy filled with regrets, and sometimes differing memories of events. We will not say anything negative about our father’s ex-wife,” they said. “He has said before, privately and publicly, that he regrets any pain he may have caused in the past to people he loves.”

Gingrich has worked in recent years to present himself as changed man, offering himself in this campaign as a 68-year-old grandfather who has settled down with wife No. 3 and embraced God through Catholicism.

Last year, he said it would be up to voters to decide whether to hold his past against him.

“I think people have to look at me, ask tough questions, then render judgment,” he said then.

But he may not have been banking on his ex-wife, who has been silent so far in the 2012 campaign, to re-start that conversation.

In the NBC appearance, Gingrich said he planned to discuss “real stories,” and said he’d have to leave questions about his character up to voters. He called his daughters “credible” character witnesses.

A message seeking comment from Marianne Gingrich was not immediately returned.