[vannevar]

From Article III: "Treason against the United States, shall consist only in levying War against them, or in adhering to their Enemies, giving them Aid and Comfort."

For example, if Russia met the definition of an enemy, and if the President compromised the interests on the country in favor of Russian interests (presumably for his own personal benefit), that would probably count. The compromise would have to be knowing, though, mere incompetence is not enough. So if the issue ever arose, Trump has a pretty good defense.