Getty Images Former U.S. Speaker of the House and Republican candidate for president Newt Gingrich speaks during the 2011 Republican Leadership Conference on June 16, 2011 in New Orleans, Louisiana.

Republican presidential hopeful Newt Gringrich's ex-wife says the candidate sought an "open marriage" arrangement after admitting he was having an affair with a staffer.

Marianne Gingrich, who was the second of Gingrich’s three wives, makes the potentially explosive charge in an exclusive interview with ABC News investigative reporter Brian Ross.

Marianne was married to Gingrich for 18 years. Toward the end of their marriage, Gingrich revealed to her that he had conducted a six-year long affair with a staffer named Callista Bisek, according to the interview. He later married Bisek.

Marianne says Gingrich asked her to share him with Bisek.

"And I just stared at him and he said, 'Callista doesn't care what I do,'" Marianne says in the interview. "He wanted an open marriage and I refused."

The affair reportedly was going on while Gingrich led the impeachment trial of then-President Bill Clinton over his affair with intern Monica Lewinsky.

The decision to air the interview just days before the South Carolina Republican primary was hotly debated inside of ABC, according to the Drudge Report and the New York Times.

One source at ABC says that the investigative unit wanted to air the story immediately, while the regular political reporting division argued for a delay. Gingrich's daughters reportedly wrote a letter to ABC imploring the network not to air the interview.

When word of the interview leaked to the Drudge Report, the internal debate came to a halt. ABC revealed Marianne's charges on its website, with a video clip of some of her most explosive statements.

The full interview is scheduled to air tonight on ABC's "Nightline" program.

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