The second of Gingrich's three wives, Marianne, told ABC's "Nightline" on Thursday that her ex-husband asked her for an "open marriage" that would allow him to carry on with Callista Bisek, the congressional aide he was bedding behind her back.
She refused, and Gingrich later married Bisek.
Marianne Gingrich has come forward now, she said in the interview, because she believes her former spouse of 18 years lacks the moral character to serve as president of the United States.
When he admitted to a six-year affair with Bisek, and made his request to remain married with a mistress on the side, Marianne Gingrich said she was stunned.
"I just stared at him and he said: 'Callista doesn't care what I do,'" she said in her first television interview since their 1999 divorce.
"He wanted an open marriage and I refused."
Marianne Gingrich, a self-described conservative Republican, added she was shocked to learn he'd conducted his long affair with Bisek "in my bedroom in our apartment in Washington."
"He always called me at night," she recalled, "and always ended with 'I love you.' Well, she was listening."
In an interview last year with Esquire, Marianne Gingrich offered some insight into her ex's public and private proclamations.
"He believes that what he says in public and how he lives don't have to be connected," she said. She also told a reporter in the 1990s that she could end her husband's career with a single interview.
Marianne Gingrich is levelling her allegations as a batch of new polls suggest Gingrich is gaining ground on front-runner Mitt Romney in South Carolina.
The state's Republican primary voters, 60 per cent of them evangelicals, are weighing their options in advance of Saturday's contest.
Gingrich has attempted to portray himself as the socially conservative alternative to Romney, who's been married to the same woman for 42 years. It's a reinvention that has often seemed laughable, in particular when Gingrich has opined about the sanctity of matrimony in explaining his opposition to same-sex marriage.
The country's leading evangelicals apparently haven't bought it, recently snubbing Gingrich to throw their support behind former Pennsylvania Sen. Rick Santorum. They cited Gingrich's personal baggage.
Marianne Gingrich has said Gingrich proposed to her within weeks of meeting her and before his 1981 divorce was final from his first wife, his high-school geometry teacher seven years older than him. Newt and Marianne walked down the aisle six months later.
But 18 years later, Gingrich was carrying on an extra-marital affair with Bisek while pushing for former president Bill Clinton's impeachment over the Monica Lewinsky scandal.
Marianne Gingrich claims he asked Bisek to marry him before he'd asked her for a divorce, a request she claims came in a phone call to her at her mother's house as he was on his way to make a speech on family values and religion.
Even before the air date for the interview had been officially confirmed early Thursday, an internal Gingrich family feud had broken out as news of the impending interview caused a revisiting of a tawdry, complicated personal history that would do "The Jerry Springer Show" proud.
Gingrich's two daughters from his first marriage came to their father's defence, suggesting Marianne Gingrich can't be trusted given the emotional toll 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," they said.
"We will not say anything negative about our father's ex-wife. He has said before, privately and publicly, that he regrets any pain he may have caused in the past to people he loves."
But their mother, the first scorned Gingrich wife, has gone further, saying whatever Marianne Gingrich has to divulge might not be accurate.
It's an admirable helping hand from Jackie Battley, the woman once dismissed by Gingrich as "not young or pretty enough to be the wife of the president. And besides, she has cancer," according to L. H. Carter, a former campaign treasurer for the politician.
Gingrich has long denied allegations that Battley was in her hospital bed recovering from cancer surgery when he asked her for a divorce. His daughters have backed up those denials.
But Battley herself had this to say in a 1985 interview with the Washington Post: "He wanted to discuss the terms of the divorce while I was recovering from the surgery. To say I gave up a lot for the marriage is the understatement of the year."
On a campaign stop in South Carolina on Thursday, Gingrich refused to comment on his ex-wife's allegations, branding them "tawdry and inappropriate."
"I'm not going to say anything about Marianne," he said, the third Mrs. Gingrich standing behind him.
Even some good news for Gingrich on Thursday came with a bit of a back-handed compliment.
Texas Gov. Rick Perry announced he was dropping out of the presidential race after a series of inelegant stumbles and bumbles.
He endorsed Gingrich, but not without some reservations.
"Newt is not perfect, but who among us is?" he said.Suggest a correction