Slippery Slope Arguments in History: The 1950s and Restrictions on Communist Speech

From Justice Frankfurter's concurrence in Dennis v. U.S. (1951):

It is true that there is no divining rod by which we may locate "advocacy" [which Justice Frankfurter thought could be punished in this case, involving a ban on conspiring to advocate violent overthrow of the government]. Exposition of ideas readily merges into advocacy…. Even though advocacy of overthrow deserves little protection, we should hesitate to prohibit it if we thereby inhibit the interchange of rational ideas so essential to representative government and free society.

But there is underlying validity in the distinction between advocacy and the interchange of ideas, and we do not discard a useful tool because it may be misused. That such a distinction could be used unreasonably by those in power against hostile or unorthodox views does not negate the fact that it may be used reasonably against an organization wielding the power of the centrally controlled international Communist movement. The object of the conspiracy before us is so clear that the chance of error in saying that the defendants conspired to advocate rather than to express ideas is slight….