I was one of those who rolled my eyes when TFG announced his run for the presidency in 2015. I thought he was a ridiculous buffoon and couldn't imagine him getting very far. I did not take him at all seriously, and once the debates started, my opinion of him kept dropping lower and lower as he said and did one ridiculous and horrifying thing after another.
As time went on, things got weirder and weirder. And then....he actually nailed the nomination.
I knew I couldn't keep dismissing him outright once it became clear that his name was going to be on the ballot in the general election. Still, I didn't see how he could possibly get elected. But we all know what happened, and I was absolutely gutted (and totally baffled) by the 2016 election results. Of course, we now know that he had loads of help from Putin and his bot farm minions, but at the time it just seemed inexplicable.
The rest, as they say, is history. His entire presidency was like a trip down a rabbit hole to a place that was a cross between Dante's Inferno and a carnival sideshow. So yeah, 2016 was when it felt like everything changed, for me. There were certainly earlier signs that things were headed in a bad direction, but 2016 was when it felt REAL to me that something crucial was, for lack of a better word, broken.