Senator McCarthy was the most prominent seeker of Communists in the federal govt in the late 1940s and early 1950s. He gained tremendous popular support for a while until he was undone by alcoholism, poor television presence, and his relentless enemies.
He was on to something. Back then, thanks to poor news coverage and lack of practical experience, it was possible for reasonable people to believe Communist propaganda promising a better way. Such beliefs inspire people prone to political activism, and such people are also drawn to govt positions. So it was no surprise to find that there were numerous govt employees who as a minimum had Communist sympathies.
Now why would fighting Communism be a partisan issue? Because Democrats had run the govt lock stock and barrel for well over a decade and had a lot to hide. And then as with Bill Clinton they tried to frame the whole affair as a "witch hunt". It's not that they didn't know there were problems - Roosevelt had already sacked Henry Wallace in 1944 in favor of Harry Truman because of his left-wing sympathies. They just didn't want to be held accountable.
Much of the complaining about McCarthy was about his "methods". Perhaps the worst of these was dredging up past associations with Communists or sympathizers, by which standard even Ronald Reagan could be presented as a Commie. People's philosophies change over time as they gain knowledge and experience. So it is questionable to bring up things that someone 10 or more years before if there is no evidence of continuity in the meantime.
One might also have argued at the time about the degree of threat Communist govt figures posed. That verdict is now in:
After the 1950s the Soviets no longer had as large a cadre of “Soviet patriots”-Western Communists-on hand for espionage. Western counterintelligence operations, the discrediting of the Soviet Union during the Cold War, and Khrushchev’s “secret speech” denouncing Stalin combined to dry up the pool of espionage talent that had proliferated during the 1930s and 1940s. In the Cold War period (1946 to 1991) the Soviets were forced to rely on less trustworthy and less dedicated mercenary agents. The loss of most of their ideological agents-one of their most valuable assets-was a blow to the Soviets.It's fair to argue about how much Senator McCarthy contributed to this, and some might even say he actively discredited anti-Communism. But we can say that there were Communists in the govt, some of them were disloyal to the US, and much was done to get rid of them over the McCarthy era.
His cause may be gone, but his methods live - more later.