In a parallel universe, Mr. President was the current President of the United States.
History
After the murder of President Lex Luthor, a new President came to power through unknown means. However, the Justice Lords begun their fascist rule over earth, thus leaving the new President nothing but a possible figurehead. Two years later, the President asks the Justice Lords if he can "elect" a successor, which Superman countered by saying "it just isn't the right time, Mr. President".
Background information
The depicted subject bears the likeness of the (then) real-life President George W. Bush; however, the DCAU character was never named on-screen, or otherwise. It is never mentioned whether he was Luthor's Vice President, or a puppet "elected" (or "selected") in his stead.