Wikipedia's list of characters in Atlas Shrugged describes Mr. Thompson as follows:
Mr. Thompson is the "Head of the State" for the United States. He is not particularly intelligent and has a very undistinguished look. He knows politics, however, and is a master of public relations and back-room deals. Rand's notes indicate that she modeled him on President Harry S. Truman, and that she deliberately decided not to call him "President of the United States" as this title has "honorable connotations" which the character does not deserve.
In what way was he based on Harry S. Truman? Why Harry Truman in particular?