Politically speaking, Sen. John Ensign of Nevada is about as battered as it gets. The Republican senator in June acknowledged an extramarital affair with a campaign aide, who was married to his best friend, who has castigated Ensign in television interviews as a shameless Lothario. In recent days, after the New York Times reported on Ensign's efforts to silence his mistress' husband, Doug Hampton, the Senate ethics committee launched an investigation and talk surfaced of a possible FBI inquiry.