Every time I think I've found the first mover, there's a justification that goes further back. I'm a solid left-liberal, represented most closely by Obama, who discovered through the left's increasing embrace of dishonesty/bigotry/other dirty tactics that his liberal identity was more important than his leftist one. This was years and years before Trump ever entered the scene, and I know people on the right who consider Trump almost as much of a monster as I do, but also consider him to be a necessary fight-fire-with-fire evil against the left's dirty tricks.
And of course, many leftists who embraced illiberalism in that period can reasonably point to something that GWB did as their justification for further weakening norms. And so on and so on ad infinitum.
Anybody with a brain and a history book understands that that's a positive feedback loop that's caused untold amounts of suffering over the course of history, and the social technologies we call liberalism smooth out the tendencies of this cycle to escalate and sublimate the "kill everyone you disagree with" impulses of most of history into progressively gentler forms of resolving conflicting ideas of how the world should be, while still allowing for progress.
That isn't to say that norms and institutions are always sacrosanct and that sharp breaks are never a good idea, but thinking that this started with Trump betrays a misunderstanding of the way these things work and why liberalism exists in the first place.
I often think of the election of Donald Trump in this sense...in a way, he is showing us crystal clear how our political dialogue, if not our entire perception of reality, is illusory. Who would have thought that someone could run for president largely on a platform of obvious lies and one-dimensional platitudes? Well, now we know.
And rather than the left being shocked into a higher state of consciousness and considering they may need to rethink their overall political strategy, instead it seems they've essentially decided to double down with more of the same theater. I hoped there would have at least been some serious consideration of a strategy consisting of campaigning on the actual truth (to the best of their ability), rather than just "more true".
So many people, particularly those on the left I would say (the right tends to be in their own little world), think politics is based on a battle of facts, when really it is a battle of memes. It doesn't have to be, but someone righteous has to be allowed to make it through the Democratic primaries if we hope to ever move forward in this respect.