The reason I say "borderline" is because Gianforte has not yet been elected to congress. If he were than it would be unconstitutional. "Congress shall make no law respecting an establishment of religion, or prohibiting the free exercise thereof; or abridging the freedom of speech, or of the press; or the right of the people peaceably to assemble, and to petition the Government for a redress of grievances.

I would say if the founders forbade congress from passing laws abridging the freedom of the press, they certainly didn't want congressmen to physically attack the press.