Newt Gingrich lacks the moral character to serve as President, his ex-wife Marianne Gingrich told ABC News, saying he did not practice family values during their 18 years of marriage.

In an interview to be broadcast Thursday on “Nightline,” Marianne Gingrich, his second wife, said that Newt Gingrich asked for an "open marriage" after he admitted to having a six-year affair with Congressional aide Callista Bisek, whom he married in 2000.

"And I just stared at him and he said, 'Callista doesn't care what I do,'" Marianne Gingrich told ABC News. "He wanted an open marriage and I refused."

Gingrich's second wife described her "shock" at his behavior, including that she learned he conducted his affair with Callista "in my bedroom in our apartment in Washington."

It is unclear what impact the interview will have on Gingrich’s presidential bid. His campaign is surging in the polls just days ahead of the South Carolina primary.

On Thursday, he received the endorsement of Rick Perry, who is leaving the race, adding to his campaign's momentum.

Marianne Gingrich said she is coming forward now so voters can know what she knows about the Republican presidential candidate.

Trying to blunt the impact of the interview, the Gingrich campaign issued a statement of support from his two adult daughters from his first marriage.

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Newt Gingrich responded to the allegations during Thursday night's Republican presidential debate. He angrily denied that he asked his second wife for an "open marriage" and criticized what he called the "destructive, vicious, negative nature of much of the news media."