Or, more likely, you can do what the supreme court has done in the past with the Roth test [1], argue “you know it when you see it”, and dodge the challenge of rigorously defining the topic while still allowing for some prosecutions to occur.
Of course, that just requires people to agree whether something is "patently obscene". Now get people to agree on whether something is fair in a political situation...
"For my friends, anything. For my enemies, the law".