Dependant on what you mean by Communism and "legitimate threat".

Was there any threat of a Communist Revolution taking place? No. This potential was already destroyed during the First Red Scare twenty years earlier.

Was there any threat that the Soviet Union would invade the USA? Well no, the Soviet Union nether had the capacity for such an undertaking and the threat of Nuclear war was enough of a deterrent.

The actual threat is for #1 spot in International politics. First the overtaking of Eastern Europe, the partition of Germany. The proxy war over Korea as well as the takeover of Chinese by Mao and his party.

The threat for isolation of the USA was somewhat real. Somewhat because Yugoslavia as well as China soon split with the Soviet Union and can't be seen as either allies or even affiliates. The biggest Asian economy, Japan, was very much under control of the USA.

The battle place, if there would've been a hot war, would have been in Western Europe and Korea. The US-mainland never was under any direct threat (except Nuclear War) by internal or external Communists.