Possible Clues:

• Pres. for over 12 years
• TVA pres.
• 32nd president and this puzzle's theme
• Real-life figure portrayed in movies by Jason Robards, Jon Voight and Bill Murray, in brief
• NRA supporter
• He said ''Be sincere; be brief; be seated''
• Pres. on a 59-Across
• ''Brain Trust'' prez
• President who created 30 Down
• 10¢ picture