While working for the Department of Defense, my security clearance could be revoked for many trivial reasons. Without my security clearance, I would have been fired.
This is a good thing!
If there is even a hint that your personal activities could be used by foreign agents for black-mail, then your security clearance is immediately suspended.
I'd really have hated to lose my job over that. After all, what control did I have over it???
But McCarthy was being stonewalled. He had to push if he hoped to get anywhere.