I think it's been shown that people vote in greater numbers if they think their candidate is winning. So the pollsters took it upon themselves to tell everyone that the Democrat was winning.
And it would have worked too, except that siding with the winner isn't the ONLY thing that motivates people when they vote. Really liking one candidate plays a role. So does really despising another. While Donald Trump was pretty good, I think what swung the election was how truly awful Hillary Clinton was. She was THAT bad. Bedwetters can't see it, but everyone else can.
Of course, Donald Trump is a master at getting people to self-destruct. If you want to credit him with the win on that basis, nobody could argue.
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