Does anyone know the answer???

Obama today endorsed John Stewart's "Rally to restone Sanity" today. Has this ever happened before? I'm asking anyone if they know the anser to that. For the life of me I can't think of any president doing that. Not that its totally worng for him to do it, since it is the presidents right to his own free speech, but I'm curious to know if this is setting a precedent or at the very least a modern precedent.