This happens to be thorny question. The core existential threat facing the demos is any attempt to use political action in order to overthrow the Demos and replace it with an authoritarian regime however disguised. We saw it happen under Fascism and multiple times under Marxism. In addition it is the central threat in Islam where almost no effort is made to convert but huge efforts are made to take control of governance from which position force and coersion is applied.
In the USA, Marxist professors have steadily marketed a disguised version of Marxism in spite of the fact it collapsed so miserably. Amazingly they have allied with the Islamist and shrouded their movement under the banner of progressives.
Both Marxism and Islam have no place in the political life of a nation as we are discovering painfully in Egypt and elsewhere. Dictators have kept them at bay and other methods have also been tried. In fact political Islam needs to be outright banned.
Any political movement whose intent is to establish authoritarian rule must be banned as it is a dire threat to the voting power of the Demos.
Mccarthyism was an out of control application of power to achieve this central goal. It did shut down the discovered communist threat at the time and place but failed to establish rules for all that prevented such a movement.
No Demos will ever vote for a king and should not. Back door schemes need to be identified and confronted and likely destroyed...
Another reality: Spies did unquestionably infiltrate the U.S. federal government in the 20th century. These spies stole secrets and influenced national policy. This reality is less known and less repeated. Moreover, while the Soviet Union is history, communist spies are still among us.