The first excerpt from ABC's blockbuster interview with Newt Gingrich's second ex-wife Marianne paints the former speaker in a negative light.

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In a move that could stop Newtmentum in its tracks, ABC News plans to air an interview Thursday night with Marianne Gingrich, the second of former House Speaker Newt Gingrich's three wives. it comes at a bad time for Gingrich, who -- after being left for dead for the umpteenth time after fourth-place finishes in Iowa and New Hampshire -- has clawed his way back to contention in South Carolina ahead of Saturday's primary.

The x-factor is how damaging the interview could be. With Rick Perry dropping out of the race, Gingrich's claim to being the conservative alternative to frontrunner Mitt Romney is stronger than ever. But tawdry revelations could endanger his backing among social conservatives, and it looks like there might be some. ABC has released the first excerpt from the interview. In the brief clip, Marianne Gingrich, describes her ex-husband admitting he was having an affair with congressional staffer Callista Bisek, who is now his wife. "I said to him, 'Newt, we've been married a long time.' He said, 'Yes, but you want me all to yourself. Callista doesn't care what I do.'... He was asking to have an open marriage, and I refused.'" (On the other hand, his philandering is hardly news.)

Marianne Gingrich also bashes the presidential contender, from whom she was divorced in 1999, saying his campaign positions don't square with his personal moral decisions. ABC has a few more teasers from the interview here.