A man might have all manner of flaws and imperfections and still retain his honor, but a man who cheats on his wife breaks solemn covenants made in the presence of God and treads on that which is sacred. A politician who violates his marriage vows would, with the right incentives, betray the oath of his office and is unworthy of trust.
That's my litmus test. I don't care if it's Newt, Rudy, or anyone else. I don't care how smart, gifted, or talented they are. If they are known to be an adulterer, don't count on them to be faithful when the going gets tough. Newt lost my potential vote when it was revealed that he had been in a long-term extramarital affair while impeaching Bubba.
That might disqualify half the politicians in Washington. If so, good! We need to clean house there anyway. Just because God forgives a penitent adulterer doesn't mean we have to hand him the keys to the White House as a display of faith.