I know that advocating specific acts of violence (for example, posting on Facebook that someone should go murder a specific person) are not covered by the First Amendment. But what about mundane platitudes calling for revolution, overthrow of the U.S. government, or whatever (“The entire federal government needs to be thrown out and replaced!”), without specific incitements to violence (“grab your gun and meet me Sunday at 1600 Pennsylvania Avenue!”)?
Yates v U.S. (1957) says no. Specifically, it states that merely expressing the opinion that the government should be overthrown cannot be illegal.
Don’t do this if you ever hope to get a security clearance.
Those qualify as campaign slogans.
I can’t imagine that ever happening, but nonetheless I’ll keep it in mind. 
See also Brandenburg v. Ohio: Brandenburg v. Ohio :: 395 U.S. 444 (1969) :: Justia US Supreme Court Center
I’ve never seen a good case about what exactly is “imminent.” That could have a strong meaning or a weak one. Technically speaking, if I say that I am going to the closet to get a gun and return and shoot you, the threat is not imminent in some sense, but for other definitions of imminent, it certainly is.
It seems like under the test, one could threaten to do an act of violence on February 7, 2018 at 2:15p.m. and it be completely legal as a date nearly a month away is not imminent.
I seem to remember Peter Ustinov claiming that to the question (on the form the US authorities asked new arrivals to fill in) along the lines “Do you intend to engage in activities intended to subvert the US constitution?”, or something equally idiotic, he answered “Sole purpose of visit” and was still let in. But that was in the early 60s.
I just looked up the form since I just had to complete it again last week. It does mention “use of force” or “force” in all the questions, so you might still be okay. 
In the days before (I personally had) the internet, I made several dumb speeches in High School oral comms class advocating armed revolution against the government, inspired mostly by those giants of political thought behind the band Rage Against the Machine, as well as a book by Che Guevara I found in the library.
A couple years later I joined the Army and was granted a secret clearance with no issue. Which is now long expired, but I sometimes wonder if my extensive internet ramblings would preclude ever getting another. At least I don’t advocate armed rebellion anymore. Though my ideas are probably even more of a threat to the powers that be, would only anybody listen.
A revolution is always legal in the first person, i.e. “our revolution.” It’s only in the third person - “their revolution” - that it becomes illegal.
There’s the old joke about the little old immigrant lady taking her citizenship test, who got confused by the question, “Do you advocate the overthrow of the United States government by subversion or violence?”
After being puzzled for a moment she said, “If I have to choose, I choose violence.”
I remember filling out that question on my green card application (in a USCIS office) and wondering if they’d give me an extra copy so I could take one home with “yes” checked. I was 13, so I don’t think I would have made a very effective revolutionary.
Now, if successive King Georges had only thought to have such a form…
There was big trial of communists in the 1950s and the charge was something like conspiring to overthrow the US government by force or violence. They were convicted and the conviction was upheld. Ah, here is a link: Smith Act trials of Communist Party leaders - Wikipedia
There’s a difference between advocating and conspiring, though. If you just say “Let’s blow up the Capitol!”, then you’re just advocating, and are legally in the clear. On the other hand, if you say “Bob, you go buy a bunch of fertilizer. Frank, you get the fuel oil, so it’s not on the same credit card as the fertilizer. Jim, you rent a moving truck, and I’ll ram it into the side of the building during the State of the Union. I’ve got a disposable phone that we can use to trigger the bomb; Mary, you watch from across the street and as soon as you see the truck go through the wall, call this number.”, then that’s conspiracy, and everyone involved (except for Frank, who was actually an FBI agent who busted them) is going to prison.
Is it a violation of American law for somebody in America to call for a forcible revolution in another country? If I (an American) was calling for people to assassinate Queen Elizabeth II and blow up Parliament, would that be illegal? Suppose I was calling for the people of North Korea to rise up, kill Kim Jong-un, and violently overthrow their government?
If it’s not even illegal to advocate for it here, how could it be illegal to advocate for it somewhere else?
Because that would be foreign policy, and explicity reserved for the President?
If they enforced that, most of Congress would be in jail. Not opinion, fact.
This is one of those questions that will largely depend on the makeup of the Supreme Court, and the public pressure surrounding them, at the time that it reaches them. Likely the next time this is seriously tested en masse, it won’t be in a court of law but the court of public opinion followed by large-scale rioting. You can look at the letter of the law and believe Brandenburg v. Ohio’s “imminent lawless action” test… if you’re a raving lunatic, you’re not even worth prosecuting at the local level (unless you embarrassed some insecure politician), much less all the way up to the Supreme Court.
But if you get any sort of traction going, even if it’s just to vaguely protest capitalism (Occupy) or fracking (Standing Rock), you’ll very quickly find out the difference between the law, the people who enforce it, and the agendas that selectively enforce and pervert it. The United States has a long history of fabricating charges and half-truths to dispose of its enemies, foreign or domestic. This has always been a country driven by propaganda and charisma, not the benign, even hand of far-sighted law. You’d be an idiot or a lawyer to believe otherwise.
A government that allows itself to be overthrown tends not to last very long, but a government that is too insecure in its power tends to breed outrage and eventually collapse anyway. The United States has both one of history’s most effective monopolies on violence (the security and intelligence apparatuses are much better equipped than even the sum total of armed and informed citizens) and – perhaps paradoxically – one of its most lenient free speech systems. Rather than shoot you on your soapbox, we’ve come to learn, it’s easier just to throw up a thousand more soapboxes and lose you in the noise. People get back to work a week or two later, power and wealth are further entrenched, and revolution becomes an armchair debate on web forums.
TLDR: The letter of the law only protects you when you are of insufficient danger to the state. The letter of the law will be warped to fit the needs of the governors when you become a sufficient threat.