MARTIN LUTHER KING JR., conservative. That description of the civil rights leader whose birth we celebrate today might surprise or even offend many of the people coming to town to celebrate the inauguration of a new president and the supposed triumph of conservatism in some form or other. (The current choices seem to range from Calvin Coolidge to Marine Le Pen.) But in his way, Dr. King did a lot to preserve, protect and defend the best of our principles and values. Just as Franklin D. Roosevelt, who was despised by many conservatives of his day, helped keep American society from succumbing to the radical ideologies that brought death and devastation to much of Europe and Asia, Dr. King worked to turn back extremism, violence and racial nationalism at the height of the civil rights movement, and to keep the cause of essential and long-overdue change in the American mainstream.