The things people remember about the long-time FBI Director are probably the the stories of cross-dressing and the relationship with his aide. In his day it was different. He served as Director from the founding of the agency in 1935 until 1972. His role actually preceded the FBI: from 1924 through 1935, he headed the Justice Department's Bureau of Investigation.
He ran his own domestic surveillance agency under eight administrations: Coolidge, Hoover, Roosevelt, Truman, Eisenhower, Kennedy, Johnson, and Nixon. No President fired or reassigned him: he stayed in charge until he died in office. J. Edgar Hoover would know what the NSA story is about.
Hoover built up files on the personal lives of anyone he considered a threat to his vision of America: from Felix Frankfurter to Martin Luther King, Jr. While he was in charge, the FBI focused on his peeves and ignored others. Liberals were a threat, the Ku Klux Klan not so much.
He carefully leaked rumors about the existence of these files: that they were extensive, and that they included Presidential staffers, Congressional aides - and Senators and Presidents.
That was how he stayed in charge for 48 years. Blackmail: always implicit, and we will never know if he needed to make explicit threats.
He would not be surprised to see Senators and Presidents defending today's universal surveillance program. It would be gross incompetence to own those files, and let elected officials feel safe challenging you.