Like many popular entertainers in the 1940s and 50s, Jack met and knew (and was photographed with) the famous politicians of the era. However, Jack struck up a special friendship with Harry Truman whom Jack deeply admired.
There are several filmed exhibits at the Truman Library that highlight this friendship. In 1959, Jack, Jimmy Durante and Melvin Douglas were the only non-politician/statesmen invited to speak at Truman's 75th birthday gala. President Truman returned the favor later that same year by making a guest appearance on Jack's television show, which was filmed at the Truman Library. Jack was also present at other Truman celebrations, including the latter's 77th birthday party. http://www.trumanlibrary.org/photos/avmovie.htm
Unlike some other entertainers then and later, Jack didn't befriend all presidents or seek out their company. His life was rich and complete enough without his needing to bask in the statesmens' glow. And, thankfully, Jack never felt the need to lecture us on every political issue of the day a la today's Hollywood. His friendship with Truman was based on their shared mutual respect and reflected the simple dignity that we associate with both men.