(RNN) - The second wife of Newt Gingrich claims the GOP presidential candidate asked her to open up their marriage to a mistress.
"He was asking to have an open marriage, and I refused," Marianne Gingrich said in an exclusive interview with ABC news.
The ex-wife told ABC News Chief Investigative Correspondent Brian Ross that the woman in question was Callista Bisek, Newt Gingrich's current and third wife.
"And I just stared at him and he said, 'Callista doesn't care what I do,'" Marianne Gingrich said.
But she said she told her husband, "that is not a marriage."
The full interview, which features the first televised remarks from Marianne Gingrich since her 1999 divorce, is set to air Thursday night on Nightline.
It comes just two days before the Republican presidential primary in South Carolina. A CNN/Time/ORC International poll released Wednesday indicates that Gingrich is closing in on frontrunner and former Massachusetts Gov. Mitt Romney.
During a Thursday morning interview with NBC's Today show, Newt Gingrich refused to speak about the soon-to-be-televised interview, saying he was focused on the big issues facing the American people because of the failures of current President Barack Obama's administration.
"I'm not going to say anything negative about Marianne," he said. "My two daughters Kathy and Jackie have sent a letter to the president of ABC News saying - from a family perspective - they think this is totally wrong. They think ABC should not air anything like this and that intruding into family things that are a decade - more than a decade old - are simply wrong."
In spite of the controversy, Texas Gov. Rick Perry endorsed the former House speaker as he dropped out of the GOP race Thursday.
"I believe that Newt is a conservative visionary who can transform our country," Perry said. "We have had our differences, which campaigns will inevitably have. Newt is not perfect, but who among us is?"
The letter from Gingrich's daughters, which was obtained by NBC News, said the failure of marriage is a terrible, emotional and regrettable personal tragedy and that memories of it are often different.
"We will not say anything negative about our father's ex-wife," the letter reads. "He has said before, privately and publicly, that he regrets any pain he may have caused in the past to people he loves."
Newt Gingrich told Anne Curry that his daughters were credible sources to his character.
Marianne Gingrich, however, told ABC News that her ex-husband has never apologized, lacks the moral character to be president, and that he carried on his affair with Bisek-Gingrich at their Washington apartment.
"He always called me at night, and always ended with, 'I love you,'" Marianne Gingrich said. "Well, she was listening."
She contends her ex-husband's affair happened at the same time he was publicly condemning Bill Clinton for having sexual relations with White House staffer Monica Lewinsky.
Marianne Gingrich said the former House speaker began planning his presidential bid during their divorce, saying Callista "was going to help him become president."
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Newt Gingrich received backing Thursday from former candidate Rick Perry, but not an endorsement. (Source: CNN)
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