During the 20's and 30's not a lot but a good amount of poeple associated with helping trade unions get started and worked directly or indirectly with the US communist party. One of those people was for example my grandfather. He helped set up several Merchant Marine Unions and Longshoreman unions. When WW2 broke out he joined the navy and became a captain and fought in the pacific and the Atlantic, including getting twice torpedoed and once kamikazed. After the war he remained a captain till one day a small ship carrying a rear admiral who after a talk in his cabin about his past which he did not deny, refute, or repudiate told him that he was relieved of duty.

That's what happend duing the 50's and 60's. You where Blacklisted not neccassarly arrested for the charge of "communisim."

A great example of this was in Hollywood. A lot of Hollywood actors/writers/directors where during the 20's and 30's involved with again unions and the communist party. If you where outed as a ex communist then your career was over lest people thought Hollywood was some Red Hub of Villainy. Lots of people where out of a job because no one would work with them.

So in short the persecutions where mostly extrajudicial and more social and societal. It meant losing your clearance if you worked for the government or not being able to get a job in your field like in Hollywood.

Now if you where subpenaed by the HUAC and you refused to testify you WHERE found in contempt and then you could be prosecuted.