Was the fear of communist takeover in the 1960s a valid fear?

First of all, please note that the fear of communist
takeover of the United States is usually much more connected with the McCarthy Era of
the 1950s, not with the 1960s.  Secondly, the answer to this can only be a matter of
opinion.

My opinion is that an actual communist "takeover"
of the country was impossible.  Americans' attitudes about government and economics are
much too firmly anti-communist for such a takeover to have happened.  Americans would
never have allowed a politician or small group of politicians to implement communism. 
Even today, long after the Cold War, we can see how strongly people have reacted to
"Obamacare," which is hardly a communist takeover.

However,
it is important to note that there were communists in the United States who were
pro-Soviet.  The Rosenbergs really were spies.  Alger Hiss was likely a Soviet agent. 
There were truly communists in America, but there was no way they could ever (in my
view) have taken over the US.