Not treason by law
The US Federal treason statute is Article III of the Constitution. Here's some discussion.
A case might be made for sedition -- by someone much more expert at the law than myself, of course.
Speaking of "support for the enemy" (which is not the same as "aid and comfort", btw) ... If you can stand to read more filth from the mouths of Phelps and his cult, the Anti-Defamation League has collected a representative sample. Here's the ADL's page of the WBC on America.
Scroll down to the last item...