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 front-runner Mitt Romney.

In excerpts of the interview released ahead the ABC broadcast, Marianne Gingrich said her husband conducted his affair with Callista "in my bedroom in our apartment in Washington" while she was elsewhere.

"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, asked by a voter Thursday about his past mistakes, said questions about his past life were inevitable but that he'd long since sought forgiveness. He said he expected attacks when he got into the race.

"We knew we would get beaten up," he said while campaigning in Beaufort, S.C. "We knew we'd get lied about. We knew we'd get smeared. We knew there would be nasty ads and we decided the country was worth the pain."

Gingrich spokesman R.C. Hammond flatly rejected Marianne Gingrich's account, saying: "It couldn't be any more opposite of the truth."

He told reporters that he wouldn't say anything against his ex-wife, but added that his two daughters from his first marriage had written to ABC to complain that the interview was "tawdry and inappropriate." He didn't answer questions about the specifics of the interview, directing questions to his daughters.

In an interview Thursday with The Washington Post, Marianne Gingrich said that within days of asking for a divorce, her husband gave a speech in which he stressed the importance of ethics and family values in American culture.