South Carolina Gov. Mark Sanford's love-struck romp in Buenos Aires with his Argentine "soul mate" wasn't just a threat to his marriage, job, and presidential aspirations. It also jeopardizes his Department of Homeland Security clearance and raises new questions about his candor on the steamy affair. Didn't know the Republican had one? Well, as a chief of state and head of the South Carolina National Guard, Sanford has a top-secret security status that lets him in on classified information such as possible terrorist threats and emergency tips. But with that need to know come intelligence community rules of conduct, a key one being that relationships with foreigners must be revealed. The reason: Those in the know can leave themselves open to blackmail from rival intelligence services about a compromising dalliance.