From Dial B For Blog:
The story revolves around Superman encouraging the youth of America to join Kennedy’s “Presidential Fitness Program.” This program was a nationwide campaign to encourage physical fitness in America’s young people. It was prepared “in close cooperation” with President Kennedy, and originally scheduled for publication in Superman #168, cover-dated April 1964. However, Kennedy was assassinated in Dallas, Texas, on November 22, 1963, and DC decided not to run the story.
(DC eventually did run the story later.)