JUSTICE DEPARTMENT LAWYER John Dion has worked on every major national security case of the past quarter-century, including the prosecutions of Aldrich Ames and Robert Hansen. Since 1997, he has run the counterespionage section within the Criminal Division. About once a week, the CIA advises his office of another case involving the unauthorized disclosure of classified information. In July, the CIA asked Dion’s office whether someone’s leaking the name of a covert CIA agent to columnist Robert Novak might have violated a 1982 law forbidding the intentional disclosure of a clandestine agent’s identity. Dion has since commenced an investigation. It is hard to imagine a lawyer more qualified than Dion to head up the investigation. But Democrats in Congress are impatient with the idea of a Justice Department investigation.