Embattled Sen. John Ensign's admission of an affair with the wife of a staff aide made him vulnerable to blackmail by hostile spy services or other interests eager to pry secrets from his position on sensitive national security committees, veteran counterintelligence officials say.

Ensign is a member of the Senate Homeland Security and Governmental Affairs Committee, including its Permanent Subcommittee on Investigations, giving him and his staff access to extremely sensitive national defense information.