Gingrich ex-wife: Newt wanted an open marriage

COLUMBIA, South Carolina (Reuters) - Newt Gingrich's second wife said in an interview the Republican presidential candidate asked her to have an "open marriage" during his affair with another woman in the 1990s, a statement that might damage his chances in South Carolina's primary vote.

Gingrich's second wife Marianne spoke to ABC News' Nightline

in an interview to be aired on Thursday.

Gingrich, former speaker of the U.S. House of Representatives, is rising in the polls ahead of Saturday's vote in South Carolina in which he is campaigning as the conservative alternative to front-runner Mitt Romney.

But thrice-married Gingrich has been attacked by his opponents for having too much baggage, much of it from his personal life.

His second wife, Marianne, said in the interview that he asked her in the late 1990s to tolerate an affair he was having with congressional aide Callista Bisek, who is now his third wife.

"He was asking to have an open marriage and I refused," she said. She has spoken publicly before about Gingrich's behavior during their 18-year marriage but the timing of the comments to ABC may damage him.

Gingrich is moving up on Romney in the polls and hopes that conservative evangelical Republicans will rally around him to defeat the more moderate former governor of Massachusetts.

Republican candidates are in a state-by-state contest to select a nominee to face President Barack Obama, a Democrat, in November's election.

At an event on Thursday in Beaufort, South Carolina, Gingrich was asked by a voter about his character.

"I have been very open about the mistakes I've made," Gingrich said. "I am 68 years old. I am a grandfather."

Two of Gingrich's daughters from his first marriage, Kathy Lubbers and Jackie Cushman, defended their father in a statement Wednesday night.

Texas Governor Rick Perry, who suspended his campaign Thursday, endorsed Gingrich and spoke of forgiveness and redemption.

"Newt is not perfect but who among us is?" Perry said.

(Reporting by Samuel P. Jacobs, additional reporting by Steve Holland in Beaufort; Editing by Vicki Allen)