Politically correct is an adjective. It is a way of describing speech
or action as conforming (or, as in "not PC", not conforming) to the
established beliefs of whatever group holds power within the context
of that speech or action. In a conservative town like Irvine, it is
not PC to say that affirmative action is a good thing. However, within
a faculty meeting in Humanities inside UCI, the PC polarity is the reverse.
If we were living in a democracy, PC would be those beliefs held by
the majority, by definition.
Republicans (and other assorted wackos) have a nasty habit of turning
normal words into insulting slang terms, based primarily on the theory
that no one in their constituency will bother to think about what they
are saying as opposed to the way they say it. Calling something PC is
therefore just another way of saying you have run out of any rational
arguments to counter someone else's opinions.
Just like associating something they said with some aspect of
the Nazi party.