Problem: After an attempted bombing of an abortion clinic, the city where Walter lives passes an ordinance that forbids all forms of protests against abortion clinics. Walter wants to hold a political rally on the other side of town in favor of a proposed partial-birth abortion ban pending in the state legislature. However, he is not sure whether his protest would be illegal under the new city ordinance, so he cancels the protest. Walter wants to challenge the new ordinance in court. What is the best legal theory might he use to challenge the ordinance?