When did American politics hit the tipping point?
Before or after the current occupant of the WH was elected?
Or have we reached the tipping point?
IMHO, this is where the answer lies for those wondering what has happened.
While I understand your question (the top one), I think it's the wrong question.
To use analogy, I think that question works the same way that someone evaluating his or her divorce wants to know, which is, "What is the one thing that could have been done differently to change the outcome?"
That question ASSUMES there is a singular point, and I don't think reality works that way.
I can give a list of problems that both political parties caused that brought us to where we are now, but I would hasten to add that at least SOME of them (e.g. moving delegate selection for nominees from state conventions to primary or caucus voting) were very good intentions that had unforeseen consequences. To give another example, the modern public financing was the result of Watergate - and the result has been candidates winning nominations primarily on the basis of their abilities to raise money (and thus poll numbers), which has nothing at all to do with governing.
The thing I never cease to point out, though, is you have to remember that Trump is an EFFECT of all those things that happened before, not a cause. In all seriousness, part of the reason so many folks pay so little attention to his marathon of lies is because they cannot name a single politician who didn't lie to them about something big - from U2 spy planes to keeping your own doctor. The public as a whole has an earned cynicism about elected officials and has learned to take any promise from a politician about as seriously as a guy who wants to score with a girl telling her he loves her and all that other seduction nonsense.
I'm obviously not saying this makes Trump's incredible non-stop cycle of lies okay - it doesn't. But you now have a public deadened to such things. The lie will only matter to folks when they observe up close and personal where it affects them.
I came up with a list of about 25 things last night that during my lifetime were big contributing factors - but I'd be provincial and self-centered to limit myself to those factors. Vietnam and Watergate were mammoth causes mostly because TV was in the picture. McKinley lied, too, but he didn't have to worry about CNN putting up video of him saying one thing and then the opposite, either.
Ultimately, it comes down to human nature and our self-centered ways.
