1. Immigration. This is the biggest reason by far. No other issue has such a huge disparity between the people and the politicians.
2. Trump's gaffes at least showed that he wasn't another scripted politician pushing focus group approved talking points 24/7. It was seen as crude, but genuine.
3. The media discredited itself. The inability to maintain even a semblance of impartiality reduced caused many people to tune it out completely even when there were fair criticisms to be made.
4. The establishment regarded the people and the issues faced in Middle America with condescension, if not outright contempt, and deliberately tried to subvert them. From the Colorado GOP refusing to have a primary so Trump wouldn't win delegates, to the DNC and media colluding against Sanders while super-delegates made it impossible for him to win, to National Review saying white working class communities should die, to Hillary calling 1/4 of America deplorable and bragging about how she would put coal miners out of work, it was a recipe for backlash.
This. The only thiing I would add to this is the fact that most Americans agree with Trump on trade.
Trump positioned himself (A) right in the middle of the electorate as a whole (B) in a place where a significant number of Republicans were at, but who could not find another Republican who really represented their issues. It kills folks to admit this, but Donald Trump has his finger on the pulse of America moreso than any other politician since Reagan. The Trump Deniers that populate Atlas can't bring themselves to admit that not only is Trump not an idiot, he's a lot sharper than they are.
Trump keeps winning. He wins ugly at times, but he wins. He's winning now, remaking the GOP to conform to HIS platform (something Jimmy Carter failed to do, and it cost him a 2nd term). Trump is all in as a Republican, and going about it the way the leader of his political party ought to be. He "gets it" far, far more than his enemies will concede. Frankly, I don't think you can stop a man who is as underestimated by his enemies as Trump continues to be.
Trump isn't moving the GOP to him, the GOP establishment are simply less principled and temporarily using him and his base for their own gain (Ted and Tip O'Neil were too stubborn for their own good on this one). And anyway, given what he advocated for that got him into The Midwest and his actual policies, it's pretty clear that's he's the one who moved more to them anyway.
Also, similar things were said about Obama after 2010 by his supporters...turns out he just propped himself up over 2012 and the party died after that.
Given where things are now, I don't see how this won't happen to Trump too, especially since the base reaction time is even faster. And that's assuming 2020 ends up in his favor.
