Read the Venona transcripts, it proves it. Also, the Russians admitted as much after the fall of the Soviet Union. Honestly, this is basic history, the only reason it isn't covered the way it should be is because of the left controls academia and has been soft to communist. McCarthy was also relentlessly attacked by the media for his anti-communist statements in the 1950s. Because of this, McCarthy is viewed as some paranoid nut who overreacted to the fact that there were Russian spies in the United States government (backed by FBI investigations). Today the red-scare is viewed as a bad event in history; however, it was 100% justify considering the Soviets and communist would eventually end up killing tens of millions around the globe.
I won't lie about a couple things. Yes there may have been communists in the U.S Government, yes the Soviets and Chinese were some of the most oppressive and horribly run countries in modern history.
However, that does not allow for his intrusions into the privacy of regular citizens to go undermined. Listen to Paul Robeson's reply to a committee that was investigating him. His answer aligns with my views on how some people felt about the McCarthy regime and their harsh intrusion into many peoples lives.
I think the main thing if not for the investigation itself is the growing anti-communist witch hunt manner that was formed in the American public during those times. In that people were being encouraged to turn in their friends for being communists. Now looking for Soviet spies is one thing, but you should be free to believe what you want and if your a communist or socialist you shouldn't have had to been subjugated to anti-soviet/anti-communist hysteria.
To conclude, McCarthy may have done some good but, it restricted left wing political freedom no matter what your opinion on the left may be everyone should be able to express their political views without being deemed criminals. *once again just to say it, actual spies and revolutionaries being investigated is okay just not regular folk.*