Johnson had three U.S. attorneys general: Robert Kennedy, Nicholas Katzenbach, and Ramsey Clark. Kennedy had served as his brother’s attorney general, but he and Johnson did not get along. Kennedy left the position to run for a Senate seat in New York. Nicholas Katzenbach had served as a deputy attorney general under Robert Kennedy, and then succeeded his former boss. He left the position in 1966 to become undersecretary of state. William Ramsey Clark, the son of U.S. Supreme Court Justice Tom Clark, served as Johnson’s third U.S. attorney general. He was known for his staunch defense of civil liberties.