Wee-hour musings lead to questions

I've recently been wondering something a little strange.

Is it a "crime" to threaten to kill a former president who has already died? Say, if some writer started calling for the assassination of "Murderous Abe" Lincoln, "The American Lenin", would the jackboots show up at his or her door to kidnap the writer?

What if the jackboots were suspicious that the writer was actually referring to a living tyrant instead of the specifically named dead one, in spite of no truth to the suspicion?

It's just a question that has been hanging around in my skull. Nothing sinister here.