If you work for any intelligence agency of the US but, your loyalties are to a communist country then, you are a security risk. Just for the fact that you disagree with the form of government of the US would give you all the incentive to sabotage and try to destroy the US from within! That fact alone should disqualify anyone whose loyalty is not to the US! All US citizens swear to defend the US from within and outside threats so, why should working for the intelligence agencies handling classified data exempt anyone?
The other thing that gets goofy is that he said- according to the wiki article that Mr. TJustice quoted above- that he voted for Gus Hall the Commie in 1976 to protest "against partisanship of the Watergate era- whatever that means. To me it means- yes I can see unhappiness with the Nixon era and wanting to go with a liberal if you are a liberal- but helloooooooo- Carter and Mondale were on the ticket in 1976. Apparently that was not lefty enough for him.