This is the third and final article in a series which has dealt with the fatal gap between the presidency and the Congress. Professor of political science at Williams College who served as combat historian in the Pacific during the war. JAMES MACGREGOR BURNShas been active in Massachusetts and national politics since his college days. He is the author ofROOSEVELT : THE LION AND THE FOX ; his new book,JOHN KENNEDY : A POLITICAL PROFILE,was published in January by Harcourt, Brace.