True, standards for real estate signs and political signs are, by definition, regulated by content, but I think this is one area where even the most careful attorneys would feel comfortable with stepping into content. (I worked on several sign ordinances with a nationally recognizd sign expert and he didn't have a problem with singling out political and real estate signs as separate sign types). However, when you do regulate political signs, its a good idea to define them as signs related to any political issue, election, referendum, etc. Don't define them as signs related to an election for a specific candidate. Also, its safer as far as potential litigation to not say that they have to be taken down X number of days after the election, referendum, etc.
Its also good to have a blanket statement saying that a noncommercial message may be substituted on a sign where there would normally be a commercial message. This substitution, though, does not waive any sign area, height, or other requirements.