Nate Cohn, New York Times, November 9, 2016

Donald J. Trump won the presidency by riding an enormous wave of support among white working-class voters.

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{snip} Those polls showed that white voters without a degree were now just one-third of the electorate. It was interpreted to mean that there was not much room for additional Democratic losses, especially once a white Democrat replaced Mr. Obama on the ballot.

The truth was that Democrats were far more dependent on white working-class voters than many believed.

In the end, the bastions of industrial-era Democratic strength among white working-class voters fell to Mr. Trump. So did many of the areas where Mr. Obama fared best in 2008 and 2012. In the end, the linchpin of Mr. Obama’s winning coalition broke hard to the Republicans.

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The rural countryside of the North swung overwhelmingly to Mr. Trump. Most obvious was Iowa, where Mr. Obama won easily in 2012 but where Mr. Trump prevailed easily. These gains extended east, across Wisconsin and Michigan to New England. Mr. Trump won Maine’s Second Congressional District by 12 points; Mr. Obama had won it by eight points.

These gains went far beyond what many believed was possible. But Mr. Obama was strong among white working-class Northerners, and that meant there was a lot of room for a Democrat to fall.

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In this election, the polls will not end up being off by very much nationally. Indeed, Mrs. Clinton will almost certainly carry the popular vote–perhaps by more than one percentage point. The national polls gave Mrs. Clinton a four-point lead in the final stretch; the final New York Times/CBS News poll had Mrs. Clinton up by three.

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But the polls were wrong about one big thing: They missed Mrs. Clinton’s margin in the Midwestern states, like Wisconsin, Michigan and Pennsylvania.

The exact mechanism for the error is unclear. Perhaps undecided voters broke for Mr. Trump; maybe there really were “silent” voters for him, people who were reluctant to tell pollsters that they backed him. Perhaps it took a lot breaking Mr. Trump’s way: Maybe Republican voters came home to the party over the last week in well-educated suburbs, while undecided white working-class voters broke for Mr. Trump.

{snip} The Clinton team, which ran its own polls as all campaigns do, was convinced it was on track to victory. It barely even aired advertisements in Wisconsin, Minnesota and Michigan.

In the end, many of the factors that made Mrs. Clinton appear favored to win in these states simply weren’t there. She didn’t win heavily Hispanic counties in Florida by the wide margins that many expected–only slightly outperforming Mr. Obama in Miami-Dade County and the Orlando-Kissimmee area, even as she outperformed in Texas and California. And she didn’t overperform in the Philadelphia area, even as she posted huge margins in the Chicago area and Seattle.

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