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.

The Supreme Court has ruled twice that destruction of the American flag is protected by the Constitution, specifically the First Amendment's protection of free speech, even if the act is unsettling.
One of the staunchest defenders of the decisions, and a key vote in favor of both, was conservative Supreme Court Justice Antonin Scalia, who was widely praised by Republicans after his death in February, including by Trump. 
Source-CNN JUly 21, 2012