Due process requires that a criminal statute provide adequate notice to a person of ordinary intelligence that his contemplated conduct is illegal, for 'no man shall be held criminally responsible for conduct which he could not reasonably understand to be proscribed.' United States v. Harriss, 347 U. S. 612, 347 U. S. 617 (1954). See also Papachristou v. City of Jacksonville, 405 U. S. 156 (1972). Where First Amendment rights are involved, an even 'greater degree of specificity' is required. Smith v. Goguen, 415 U.S. at 415 U. S. 573. See Grayned v. City of Rockford, 408 U. S. 104, 408 U. S. 109 (1972); Kunz v. New York, 340 U. S. 290 (1951).