Hmmm.
From the US Constitution.
Article. III.
...
Section. 3.
Clause 1: Treason against the United States, shall consist only in levying War against them, or in adhering to their Enemies, giving them Aid and Comfort. No Person shall be convicted of Treason unless on the Testimony of two Witnesses to the same overt Act, or on Confession in open Court.
Of course, I've heard some people argue that merely criticising the US President is giving aid and comfort to the US's enemies.