You point this out in your own post. Some of the FBI actions pre-1970s are clearly not what we consider consistent with a liberal democratic state today. How certain groups of people were treated in the past is not consistent with what we consider a liberal democratic state today. BUT in all those timeframes, Americans would have considered themselves democratic (and perhaps the last, best hope for mankind). The point is you should not be so quick to discount change in American democracy.

Trump is a change agent. Even his own supporters admit this...thats why they voted for him. Do you agree that norms have been broken under Trump? I think thats pretty clear and we will see what comes from them.

I also worry that you think there is some kind of bright-line event that will clearly show that America has gone too dar down a dark path. But I don't think this is how it normally works. It is normally a death by a thousand cuts.

I also have no idea what you mean by checks and balances. The traditional checks and balances on the federal government are the separation of powers between the executive-legislative-judicial and federalism. Federalism is working somewhat, and perhaps the judiciary. But the legislature is not checking Trump. It is enabling him which is part of the worry that it is not fulfilling its institutional checks and balances role.