Probably Abraham Lincoln.
While the specific "quote" is almost certainly manufactured, the core idea could easily have been taken from Lincoln's Lyceum Address. In that speech, Lincoln warned that the mortal danger to the United States was not foreign but domestic:
At what point then is the approach of danger to be expected? I answer. If it ever reach us it must spring up amongst us; it cannot come from abroad. If destruction be our lot we must ourselves be its author and finisher.
(In recent years this has been rendered as "America will never be destroyed from the outside. If we falter and lose our freedoms, it will be because we destroyed ourselves". That is rather more similar to McCarthy's purported "quote", but post-dates his speech by decades.)
Of course, we cannot know for certain who (if anyone) McCarthy meant, since he failed to explicitly name this "outstanding historical figure". Nonetheless, it is worth remembering that McCarthy made the reference while delivering a Lincoln Day speech. In fact, he began with:
Ladies and gentlemen, tonight as we celebrate the one hundred forty-first birthday of one of the greatest men in American history ... These would be truly appropriate things to be able to mention as we celebrate the birthday of Abraham Lincoln.
It stands to reason that he may reference Lincoln's remarks.
Note that this isn't to say McCarthy was actually quoting Lincoln. It is at best a very loose paraphrase of a liberal summary of a paragraph in Lincoln's speech.