BLUFFTON, S.C. — Newt Gingrich’s second wife says he asked her for an “open marriage” to continue his affair with the woman who became his current wife.

In South Carolina Reporting on the candidates and voters in the Palmetto State.

In an interview with ABC News to be broadcast Thursday night on “Nightline,” the former wife, Marianne Gingrich, says that Mr. Gingrich informed her he was having an affair with a Congressional aide, Callista Bisek, whom he married in 2000.

“I said to him, ‘We’ve been married a long time,'” Marianne Gingrich says, in an excerpt from the interview released Thursday morning by ABC News. “And he said, ‘Yes, but you want me all to yourself. Callista doesn’t care what I do.'”

Marianne Gingrich said that was unacceptable to her, and the marriage, his second, ended after 18 years.

It is unclear what impact the interview will have Mr. Gingrich’s momentum in South Carolina, coming just days before the state’s primary on Saturday and on the same day he received the endorsement of Rick Perry, who is leaving the race.

Mrs. Gingrich gave a similar account in an interview with Esquire in 2010, when she said Mr. Gingrich asked her to tolerate his affair, but the timing of these comments, released as polls suggest Mr. Gingrich is gaining ground against Mitt Romney, have thrown a curveball at Mr. Gingrich’s campaign.

Trying to blunt the impact of the interview, the Gingrich campaign on Wednesday night issued a statement of support from his two adult daughters from his first marriage.

At his first campaign event Thursday, Mr. Gingrich did not respond to shouted questions from reporters asking about the Perry endorsement. On Wednesday evening, both men appeared at an antiabortion forum in Greenville, S.C.