> What if I urge people, in a riveting call to action speech, to kill X celebrity or Y ethnic group?

I think the former is specific enough to be legally problematic, but I was actually under the impression that the second one is technically legal? (morally awful, but legally unprosecutable)

I think you're right. It's not specific. Edited my comment.

Such speech would only place you in legal jeopardy if it contains a direct and credible incitement to violence. If you simply said something like, "Let's kill all the Elbonians!” and nothing more then it would still be considered protected speech under current US Supreme Court precedents.

That's a grey area. The prosecution would need to prove that you intended for someone to commit the murder, that you advocated its imminent commission (or at least the imminent initiation of steps toward the crime), and that you believed your advocacy of such crime was likely to lead to someone carrying it out.

https://en.wikipedia.org/wiki/Brandenburg_v._Ohio

The speech that advocates breaking the laws, but does not lead to immediate lawless action is already protected. I.e. if you say "hey you, I order you to take this gun and kill X!" then you're in trouble. But if you say something like "moral imperative declares that X should be killed" (and it's not a coded signal for an assassin but a genuine moralistic argument not intended to cause any specific action) then it's protected. That's why, for example, people marching through Oakland shouting "Death to America" were doing it in complete accordance with the law.