I'm not the original poster, but something to think about - communism isn't a political leaning the way debating about state/federal powers is a political leaning. Mainstream communist literature - Communist Manifesto for instance - directly advocates violence and rioting, to the end of placing the state under control of the workers. Marx said that condition - control by the workers - would be socialism. Socialism could then slowly dissolve until it's a classless, peaceful society, but Marx said you'd need to make a lot of violence on the bourgeois first.
I don't know, if there was a group that advocated, for instance, that they should confiscate and destroy all the property of non-Caucasians, make sure whites control everything, but that's only a necessary step to a perfectly peaceful and equal society - well, would you want the FBI to keep tabs on those people? How about if someone believed that they should kill all infidels to place society under the control of Muslims, which would then lead to a peaceful society? No, Communism isn't a political leaning, it's a very bad thing that's done very bad things.
I'm visiting Cambodia in two weeks by the way, I'm going to see the relic of one of the most literal applications of Marxist communist doctrines:
> People were often encouraged to confess to Angkar their "pre-revolutionary lifestyles and crimes" (which usually included some kind of free-market activity, or having had contact with a foreign source, such as a US missionary, or international relief or government agency, or contact with any foreigner or with the outside world at all), being told that Angkar would forgive them and "wipe the slate clean". This meant being taken away to a place such as Tuol Sleng or Choeung Ek for torture and/or execution.
http://en.wikipedia.org/wiki/The_Killing_Fields#Process
Communism is a bad thing. The legality/criminality of advocating it should be equivalent to the legality/criminality of other forms of advocating violence and destruction.