The purpose of McCarthy's hearings was to show that Communist party members (an absolute security risk) were, in fact, being employed by government agencies. The problem was the exact opposite of what you fear...they were being too lenient with whom they hired.
"Are there terrortists in this country right now, with plans to kill Americans? Of course there are. But that is not the question. The question is how much power should government have to do something about it? And will they be held accountable when they overreach and cause injury to our citizens?"
I would hope so. But, as the book points out many times, the analagous situation today would be if known Al Queda members were being employed in secure spaces in DOD, CIA, and Department Of State, and no one had a problem with it!
We're not talking here about the government going after civilians on some witch-hunt. We're talking about rooting out enemies (and like it or not, during the Cold War, Communists were the Enemy) employed within the government, which various agencies of the period failed to do, as VENONA makes abundantly clear.
McCarthy was only using his position to point this out.