Is it more subtle than 70m people in America are stupid? Ultimately not much, and that's the sad truth.
Why do you think it's the uneducated that make up Trump's biggest demographic? These people are very literally voting to make their lives worse. To increase their taxes while millionaires pay less, to get rid of affordable healthcare even though it's got huge approval ratings, to strip away workers rights and minimum wages.
How can you explain to me why a guy working minimum wage, struggling to get by, in terrible health because he can't afford insurance is voting for the people who keep his wages down, refuse him healthcare and increase his taxes without any social benefits to show for it?
The republicans have gutted the education system, invested billions in manipulating people based on targetted ads, lied constantly, jumped in bed with the religious right wing and played off people's fears.
>> Is it more subtle than 70m people in America are stupid?
Yes, very much more so.
There are some extremely bright people voting for Trump.
There are many factors behind populist movements world-wide. Probably the factors in the USA are similar/identical to those in many other countries.
The primary factor - I think - is an overwhelming sense that the majority race/ethnicity/religious belief is being undermined by "others" - different skin color, tribe, and/or religious belief. The perception is that you personally - in the majority - are being put to back of the line by government policies favoring the "others."
This sentiment is motivated by upbringing - i.e., your parents believed this, and in your formative years you were exposed primarily/exclusively to this viewpoint - and over the years your well-being has not seemed to improve; moreover, the "others" are taking your jobs, living off welfare, committing crimes and so on... or so it seems to you.
In short, a large segment of the Trump vote have been living among like-thinkers all their lives - it's natural to believe what your peers believe regardless of intelligence.
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Another factor is single-issue voters.
Strong anti-abortion - no matter what - is a Trump vote.
Strong pro-gun rights - no matter what - is a Trump vote.
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Then, there's gerrymandering - this has the effect of concentrating each party's vote into completely separate districts. Which means that successful politicians will by-in-large adopt more extreme viewpoints than would occur if a simple popular vote election were in place. To win a partisan gerrymandered district you simply need to appeal strongly to the majority sentiment in your district - compromise positions will result in failure to win election in most cases.
In the USA - congressman will be far more partisan; contributing to the echo chamber.
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Another factor is the advent of extremely populist journalism - the various opinion shows on Fox for example (as opposed to the actual news broadcasts on Fox which are actually pretty good).
This, and the rise of social media, means that a populist-leaning individual has ready access to "news" (it's actually opinion) - that directly aligns with their current beliefs. There's a ready source of positive feedback, reinforcing existing viewpoints.
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Finally, a populist figure comes into power given all of the above. First up - discredit the free press - "fake news."
The populist media outlets echo and reinforce the "fake news" pronouncements. This means that nearly everyone who was leaning towards populist viewpoints - is locked in. It's virtually impossible to change their minds - facts per se do not matter - because the facts - in their opinion - are those reported by their favorite media outlets - everything else is simply not true ("fake news").
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It's a very short step from the scenario above - to a total collapse of democracy.
Should Trump have been elected to a 2nd term in office - America might well have transformed from a democracy - to an autocracy.
It's not at all about "stupid people."