IT IS NOW nearly twenty-five years since the Greenwich Village opening of the satirical play MacBird, which portrayed then president Lyndon Johnson as the assassin of John F. Kennedy. It is fifteen years since Jimmy Carter was elected to cleanse the White House of the taint of Richard Nixon. Since these events, we have strenuously pursued the goal of virtue in American polities.  We have become increasingly sensitive to issues of ethics, morality, and character. We have taken heroic steps to make the political system more open and honest. The press corps has grown larger and more aggressive. Government itself has joined the investigative party, through devices from the independent counsel to proliferating congressional probes. Conflict-of-interest laws have been tightened. Federal campaign contributions have been limited and subjected to increased disclosure. Politicians are less beholden than ever to corrupt political party machines or congressional seniority systems.