Only a few aging historians still remember Rep. John J. Rooney, but from the 1940s into the 1970s he was FBI Director J. Edgar Hoover’s most powerful enabler. Rooney, a Brooklyn, N.Y., Democrat, led the House appropriations subcommittee that oversaw the Justice Department. He remained Hoover’s steadfast ally as presidents from Truman through Nixon came and went.

John Rooney personified an era in which congressional Democrats eagerly aided and abetted the FBI’s running amok, as the bureau surveilled political activists who...