It must be noted that the affair occurred while the couple was separated.
From FOX: The Republican senator who went public Tuesday about an extramarital affair has resigned his GOP leadership post.
Sen. John Ensign, R-Nev., stepped down Wednesday as chairman of the Republican Policy Committee, the fourth-ranking GOP position in the Senate, after holding a brief news conference Tuesday to come clean about his affair. He is not expected to resign from the Senate.
Basically, he is resigning from the "think-tank" wing of the Republican Party.