While the affair made national news in June, the role that Mr. Ensign played in assisting Mr. Hampton and helping his clients has not been previously disclosed. Several experts say those activities may have violated an ethics law that bars senior aides from lobbying the Senate for a year after leaving their posts.

In acknowledging the affair, Mr. Ensign cast it as a personal transgression, not a professional one. But an examination of his conduct shows that in trying to clean up the mess from the illicit relationship and distance himself from the Hamptons, he entangled political supporters, staff members and Senate colleagues, some of whom say they now feel he betrayed them.