Germany has learned this lesson the hard way, with a 'defensive' constitution post-1945. You don't have 100% free speech in Germany, and it is possible to make parties illegal. It's not without its issues (currently, the far-right AfD
might be banned using these laws but the whole system has been dragging its feet) but it is a lesson the US should have learned after the first Trump term.
Democracies by default assumed that all players in the system are supportive of the system itself, kind of like all early Internet protocols assumed that there are no malicious users.