Rufus Youngblood, 72, the Secret Service agent who threw himself on top of Lyndon B. Johnson during the assassination of President John F. Kennedy. Youngblood was assigned to Johnson from 1961 to 1966. During the November 1963 presidential trip to Dallas, he and Johnson rode two cars behind Kennedy's. Youngblood heard the shots, turned from the front seat, pulled Johnson to the floor, shouted "Get down!" and then half-sat and half-sprawled on top of the vice president. The feat earned him the Treasury Exceptional Service Award. Youngblood was in the Secret Service from 1951 until 1971. On Wednesday in Savannah, Ga., of lung cancer.