I welcome discussion of the legality of this threat in the comments.
UPDATE: I don’t want to research and give an opinion about whether this might be a crime, because if I were to decide that it probably is, some lefty nut case would probably misconstrue my opinion as some kind of official threat. (I always speak here in my individual capacity, and say so in a disclaimer on the sidebar — but some dimwits don’t seem to understand that.)
However, Eugene Volokh looks at the question here. He appears to reach no firm conclusion, other than that his brain hurts. But his reasoning process is interesting.