Advocacy of treason in a general sense is legal in the U.S.; Yates v. US (1957) held that "advocacy of forcible overthrow of the government as an abstract doctrine" was protected by the First Amendment, and therefore someone couldn't be jailed solely on the basis that they were a member of the Communist Party and advocated a communist revolution. (http://en.wikipedia.org/wiki/Yates_v._United_States)
To prosecute (after Yates) you'd need some additional evidence that an individual's advocacy wasn't merely abstract, general advocacy of revolution, but concrete and imminent enough to present a "clear and present danger", e.g. because of specific steps being taken to actually overthrow the government in the near future.
Similarly, a state actually seceding from the U.S. remains illegal, but someone merely abstractly advocating that a state ought to secede, as the Texas governor half-seriously did, and many "Sons of the Confederacy" type groups more earnestly do, is legal and not considered punishable as treason, assuming that the groups aren't actively planning raids of federal forts or something.
To prosecute (after Yates) you'd need some additional evidence that an individual's advocacy wasn't merely abstract, general advocacy of revolution, but concrete and imminent enough to present a "clear and present danger", e.g. because of specific steps being taken to actually overthrow the government in the near future.
Similarly, a state actually seceding from the U.S. remains illegal, but someone merely abstractly advocating that a state ought to secede, as the Texas governor half-seriously did, and many "Sons of the Confederacy" type groups more earnestly do, is legal and not considered punishable as treason, assuming that the groups aren't actively planning raids of federal forts or something.