OK, So We Can't Make "Treason" Stick...

...how about the Logan Act?

...critically, the citizen must act “without authority of the United States.” Although most assume that means without authority of the Executive Branch, the Logan Act itself does not specify what this term means, and the State Department told Congress in 1975 that “Nothing in section 953 . . . would appear to restrict members of the Congress from engaging in discussions with foreign officials in pursuance of their legislative duties under the Constitution.” That doesn’t mean Members would have immunity under the Constitution’s Speech and Debate Clause; it just means the statute would arguably not apply in the first place. Combined with the rule of lenity and the constitutional concerns identified below, it seems likely that contemporary and/or future courts would interpret this provision to not apply to such official communications from Congress....

Finally, as Peter noted yesterday, the Logan Act has never been successfully used (indeed, the last indictment under the Act was in–not a typo–1803). Although most assume this is just a practical obstacle to a contemporary prosecution, it’s worth reminding folks about “desuetude”–the legal doctrine pursuant to which statutes (especially criminal ones) may lapse if they are never enforced[.]

There's a long history here.

In 1970, as an inactive Naval Reserve Officer, Kerry traveled to Paris and talked with North Vietnamese government officials. He even stated later to the Senate Foreign Relations Committee that “I realize that even my visits in Paris,” Kerry testified, “are on the borderline of private individuals negotiating.” He got it wrong there too. Again the Logan Act does not differentiate between public or private individual. It's pretty clear on the "any" part.

That one really was treason in the aid-and-comfort sense, coming from a military officer at a time when we were deployed at war against the very people with whom he was publicly negotiating, but the man went on to a long career in the Senate and now holds the position of Secretary of State.