Fun case: in 1949, Frank Buchanan (D-PA) was appointed chair of the Select Committee on Lobbying Activities, which investigated anti-New Deal conservatives, among others. One of their targets was Edward Rumely, who refused to disclose his subscriber list... https://twitter.com/pompilivs/status/1349022687130230785

This resulted in a SCOTUS case, United States v. Rumely (1953), which ruled in his favor that congressional committees had no power to "inquire into all efforts of private individuals to influence public opinion through books and periodicals": https://www.mtsu.edu/first-amendment/article/321/united-states-v-rumely

This ended up becoming an important First Amendment case with the consequence of crippling Joe McCarthy's efforts against communist infiltration in USG. Moral is that free speech protections in a neutral liberal society inevitably serve the long run interests of communism.