Clark is seen writing the note himself while Sheridan arrives and makes his broadcast.
You can see some of the scene in this Youtube clip. His hand is clearly going from left to right on the scene, implying he's writing the repeated "the ascension of the ordinary man" himself (I've been unable to find a full segment online that includes all of the Clark footage).
Given anecdotes about his paranoia and the fact that he committed suicide moments later, it is very likely he was not in any right mind while he was writing it, or that he developed an obsessive habit as part of his mental decline. If the note was actually intended as a delivered message from some other party, or a deliberate cryptography, there is no evidence of this shown
in the show. If there is a meaning to the phrase he used, it would have been personal to Clark - possibly he viewed himself as the "ordinary man", ascended when he gained the presidency.
It should be made clear that the Senator who attempted to arrest Clark had not just the note and its "message", but also could see the President's command console on the desk in front of her. It's unlikely Clark could have hidden his commands for long even if the note was not obvious about his intentions. Unfortunately, the dialog in the script just suggests she uses the note to reach this conclusion.
As @Valorum also notes in comment, the production team was homaging other works in this depiction.