The thing is, this has happened before. Romney genuinely thought he'd won / 'unskew the polls' etc. I think it's 50/50 as to whether the Trump campaign and supporters thought he'd win last time. If you move in a bubble, a social and online bubble you think things are going one way. So a lot of Trumpists genuinely think 'Blexit' is a thing because they follow the few Twitter accounts who are part of it. Opponents know that's not teue, not from a gut feeling or confirmation bias but from the stats. So there might be inexplicable over confidence from Trump supporters that is completely sincere.
Basically this, except I also believe the Democrats and the left live on a bubble as well. There has been a big movement towards echo chambering and bubbling up of opinions with very little interaction between both sides. That is also what drives polarization.
Basically trumpists only interact with other trumpists and Dems only interact with other dems, so of course you are going to think your side will win a landslide.
Yeah. I can see this happen as well. (I think partly because I'm more left than the typical "liberal" on many issues - I'm basically a socialist in a lot of ways - but have a few points of divergence there, too, so I can't just always go with the "left" consensus, even if I broadly agree with it a lot of the time.)
I think it's driven by a combination of factors: the end of the equal time that led to right-wing domination of talk radio and thus rural areas in the pre- and early-internet eras (it still matters now - lots of the country doesn't have working mobile data or good home internet); the internet transforming "news" into being almost entirely eyeball-revenue based (it's no surprise that much of the best journalism comes from places that aren't eyeball driven, even if they have their own biases); plus the emergence of social media, that is nearly telepathy (who wants to share mindspace with ideas and people you hate?); coupled with how human brains work; plus a leavening of people who want to exploit ignorance and who can't profit from informed, critically thinking citizens; and that's how you end up with our current mess.
If you want an example of the reverse, look at 2016: at the end, it was pretty obvious to anyone being really objective, including Clinton's own campaign team, that the election was going to be far from a sure thing. But most people on the left, including a lot of folks who follow politics and elections more closely (cough, like some of us, cough) didn't allocate any real thought to a Trump victory. The Trumpists are doing the same thing, only moreso.
One final thought on the left and right "bubbling up" themselves: it neatly eliminates any far-right, far-left, anti-establishment coalition working together to overturn the existing order. Which if you are a genteel, pre-Trump us oligarch (like Jeff Bezos) is a pretty sweet deal. (Damned if I know how to deal with that, other than get the GOP out of power for a generation and then fight for control of the Dems internally.)
