I really can't think of any long established ones, the nearest I can think of is Germany in the 1930s, but that was really a quite new democracy at the time.Over history, many democracies have fallen under the sway of authoritarian strongmen and ended up in very dark places.
I guess one could attempt to make the case that there’s something about the US that makes it exempt. “It can’t happen here”.
Not that I think it will, but I think it’s naive to deny even the possibility of it.