Well it turned out that yes, the Rosenbergs WERE communist spies. Alger Hiss WAS a communist spy. etc. The Left campaigned vigorously for them and whined and cried when they were convicted and punished (this was the source of a lot of the bitterness against Richard Nixon)....but it turns out they were WRONG WRONG WRONG about everything. What they really can’t bear to admit is Joe McCarthy was right about all the commies in government - especially in the state department.
Truman defended known communists in his administration and federal employment because he feared the partisan consequences of admitting the truth. Eisenhower did the same because he believed, wrongly, that communists were all being diligently detected and removed, with McCarthy being a political opportunist taking advantage of normal delays in administrative processes.
As it happened, FBI Chief Hoover knew better, that communists were being protected and doing grave damage to the country because of lax security. To force action, Hoover leaked information to McCarty.
Even McCarty's death from cirrhotic liver failure has been exploited by the Left by portraying him as a reckless alcoholic. In truth, it seems that McCarthy's alcohol use was moderate and that he died from infectious hepatitis, a cause of liver failure that was not fully recognized and understood by medicine at the time.
The underlying question though is why the Left was and is so eager to defend communists who put the country in danger? Many volumes can be written on that subject without touching bottom on the moral and intellectual rottenness of the Left.