Via The Washington Post:
The Supreme Court has often said that the government generally may not impose content-based speech restrictions. Content-neutral restrictions, such as evenhanded restrictions on sound amplification, on blocking traffic, and the like are often constitutional; and that extends to content-neutral restrictions aimed at promoting aesthetics, such as limits on the size and quantity of signs.
But when the government restricts speech based on its content, such restrictions are generally unconstitutional. (I’m speaking here of restrictions that the government imposes in its capacity as regulator, and not as employer, educator, or speaker; and I’m setting aside the historically recognized content-based exceptions, such as for libel, obscenity, threats, and the like.) They can only be upheld if they are “narrowly tailored” to a “compelling government interest” — the famous “strict scrutiny” test, which is quite hard to satisfy. And this is true not just for viewpoint-based laws (e.g., “no antiwar speech” or “no racist speech”) but also for viewpoint-neutral but not content-based ones (e.g., “no advocacy by corporations or unions related to any political candidate”).