The current Supreme Court rulings defines what is Constitutional.
This is a very papist understanding of the Supreme Court.
Did the text of the Constitution change on June 8th, 1969? Or did our agreed-upon interpretation of that document change? Or, even, did our legal structure, as inspired by particular values we ascribe, for our own purposes, to that document change?
My main point here is simply that assertions about what the Constitution says or does are problematic because of the implication of eternal continuity. There are people alive today who lived in an America where it was perfectly constitutional to ban white supremacists from holding rallies and declaring their intention to murder people of color, and constitutional to arrest and imprison them for doing so. There is no particular reason to consider that America somehow less American.
So simply asserting that "the Constitution lets them do it" is not a real argument. You should explain why it's good that the Constitution lets them do it and why we shouldn't agitate for another change in America's understanding of the Constitution.