It's really hard to poll these days compared to how it used to be.
That being said, I think often the media & political establishment just fails to ask the right questions, or understand how "the street" reacts to what it sees.
Both Brexit and the HST referendum for example, there was a lot of this "Project Fear" type of stuff - smug dismissal of opposition, then intellectual rationalization, then bargaining, then hysterical fear, and then finally threats. None of it worked - the more The Street saw fear in the ranks of the Establisment, that more that's what the public wanted. They were voting to spite the people who pissed them off or betrayed their trust. All the lecturing by well-connected insiders won't stop it, and unleashing a parade of fashionable celebs only convinces The Street that the Establishment has no interest in taking their concerns seriously. Hence, the Establishment flailing away at Brexit or HST or Charlottetown, then losing, and then a lot of sour grapes in the media afterwards.
That's one sort of failure, and here's another kind: With Christy Clark vs. Adrian Dix, that was a case where a "coronation" was expected, and the public hates those. Adrian Dix was known as what, prior to it - as Glen Clark's right hand man, so the first chink in the armour was that he was seen by The Street as a slick political operator with questionable past ethics. The second chink in the armour was when he tired to run a positive / sunny campaign, Jack Layton style. Well the situation was just not the same because: A) nobody expected one, since this is BC and BC politics are always a knife fight; B) To defeat the BC Liberals, really all they needed to do was bring up all that scandal and so on - run on the BC Liberals' crooked record and remind the people why the Liberals (Campbell, Christy Clark) had pissed everyone off. Dix and his gang of advisors had I guess thought they could just coast to victory, looking past Christy to the inclusive blah blah blah government they would be. Not to mention that they assumed the NDP had enough environmental credibility to draw support from what would otherwise be Green voters - they don't, but you'll never convince an NDPer of that. So basically the NDP ran a less-than-popular candidate, then put away their best weapons and ran a campaign of gimmicks. It was weak stuff and they got their ass handed to them.
As this relates to Clinton's team, well, they made a lot of the same mistakes - they misunderstood the public's volatile mood; they coasted rather arrogantly through not just the main elections but the primaries before that; they took a lecturing tone and responded to difficulties with scripted hysteria. T
Seriously - they made a terrible mistake in making the campaign about personality rather than policies and governing ideals. On that front, Clinton could have wiped the floor with Trump; instead it got reduced to "Trump's a jerk, and it's somehow Russia's fault when we embarrass ourselves."
Yes, the Dems could have won, they perhaps should have won - but they approached the contest all wrong and lost.
I don't like Trump or believe in most of his policies, and it seems a serious chunk of his own supporters don't either. But he's a torpedo into the side of the US elite, and that's honestly all the voters want. He speaks the language of the street, and promises to slay all the sacred cows of the Establishment. Godzilla comes to DC. Does he actually mean it? I doubt it. Seems like a putz, and a guy with an ego that big will only serve himself.
The Democrats should ask themselves how they managed to alienate the US working class so badly that a billionaire egomaniac scooped up all their blue collar voters. I do not think it is a matter of policy - if you look, there is support for public health care, higher wages, more taxes on the wealthy, dealing with climate change. So policy is not the problem, it's political culture - they do not know how to relate to ordinary working class people anymore. I think their party brass did a lot to alienate their own grassroots by actively taking sides against Sanders, freaking out about Wikileaks, etc.
Bernie Sanders would have been able to defeat Trump - bet your ass on it - because he is a balls-out street fighter who attacks the elite, instead of relying on them.