Active socialism could not be distinguished from communism. Communism could not be distinguished from Stalinism. Therefore the political deduction was made, that anyone opposed to the right wing establishment of the day had to be actively planning for the Russian takeover of the USA. Anyone who would deduct otherwise was a communist traitor themselves, and subject to the same witch hunt .
What always amazed me was that these inquisitors professed to have Christ on their side. Anyone who suggest that the rich should be required to share with the less fortunate were branded a communist. Seems that in the Bible Christ gave that same direction to his followers. What a paradox! If Christ were alive at the time there is little doubt that he too, would have been branded as being a communist traitor.