WASHINGTON, D.C.

Forget the attack ads up to this point. The GOP nomination race is about to get messy.

Newt Gingrich's second wife, Marianne Ginther, was scheduled to expose details of the couple's 1999 divorce in an interview with Nightline on ABC Thursday night.

In a preview of the interview on ABC's website, Ginther says Gingrich wanted an "open marriage." This was at the same time he was carrying on his second extramarital affair at the time with his current wife, Callista Bisek.

"I said to him, 'We've been married a long time.' And he said, 'Yes, but you want me all to yourself, Callista doesn't care what I do,'" she tells the interviewer. "He was asking to have an open marriage and I refused."

"He wanted an open marriage," ABC's Brian Ross said.

"Yeah, that I accept the fact that he has somebody else in his life," Ginther said.

"And you said," Ross asked.

"No," she said. "That is not a marriage."

Gingrich and Ginther began their relationship while Gingrich was married to his first wife Jackie Battley. He divorced her in 1980 while she was fighting cancer, and married Marianne a year later.

Then in the mid-1990s, Gingrich began an affair with Bisek, a Congressional staffer 23 years his junior.

Gingrich continued his affair with Bisek through the 1990s, all the while Gingrich was leading the charge to impeach then president Bill Clinton for having sexual relations with Monica Lewinsky.

Gingrich and Bisek married in 2000. In 2007, Gingrich became a Catholic.

Ginther has revealed details of their divorce before, most recently in an Esquire article last fall.

She reportedly wanted to set the record straight again as Gingrich is fighting to be the Republican nominee to take on President Barack Obama in November's election.

On Thursday, Gingrich said in a television interview his divorce is a personal matter and ABC should not show the interview. He also said his two daughters have written ABC asking it to cancel the broadcast Thursday night.