In OTL, Thomas Dewey narrowly defeated Harry Truman in 1948 and appointed J. Edgar Hoover Attorney General. This was part of a secret deal Hoover had made with Dewey to feed him dirt on his opponents in exchange for the AG spot. Soon afterwards, Hoover was appointed an Associate Justice of the Supreme Court. Hoover had hoped to become Chief Justice, but Dewey (who hated Hoover) had no intention of making this happen. Stripped of his political power, Hoover could no longer leverage the President into doing what he wanted and he remained on the Court until his death in 1972.
But what would've happened to Hoover if Truman had defeated Dewey?
But what would've happened to Hoover if Truman had defeated Dewey?