The Red Scares and the McCarthy hearings did NOT do a good job of peeling off the bad from the good. Moreover the Soviets (and even Mao himself to some extent) were our ALLIES, not our enemies. When we burned (electrically) the Rosenbergs for what was popularly called treason (the crime was espionage, not treason) that was wrong because they spied for an ALLY.
As ruthless a mass murderer of a Marxist as Stalin was, he was our ally. It is NOT a hanging offense to spy for an ally.
In order to peel off the good from the bad, the fight has to be closer in, wrestling sweat-to-sweat rather than spear chucking wildly from up the hill, otherwise what has happened is what happens. The poisonous vine gets inside the bark, and we get a rudderless self-doubting pretender or self-hater in command, we are (possibly) then sailed right into the arms of our true enemies.
This is very relevant to this thread and today since CPUSA is MoveOn.org.
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Archived screenshot of the Communist Party's website. Notice the blue text on the blue background in the left border at the bottom.