Dredging up a past that Newt Gingrich has worked hard to bury, the GOP presidential candidate’s ex-wife says Gingrich asked for an “open marriage” in which he could have both a wife and a mistress.

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In an interview with ABC News’ “Nightline” scheduled to air Thursday night, Marianne Gingrich said she refused to go along with the proposal 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 known for its strong Christian conservative bent, and as Gingrich was trying to present himself as the strongest alternative to GOP-front-runner Mitt Romney.

In excerpts of the interview released by ABC, Marianne Gingrich said Gingrich conducted his affair with Callista “in my bedroom in our apartment in Washington.”