John Hagner, a partner at Clarity Campaign Labs, a D.C.-based Democratic analytics firm, said 2016 taught the party a hard lesson about polling in the Trump era.
“The folks who would talk to a stranger about politics just aren’t representative of people who wouldn’t,” he said.
The first evidence of the party’s polling blind spot surfaced in a governor’s race, the 2015 contest in Kentucky. Both public and private polls going into the election showed Democrat Jack Conway and Republican Matt Bevin running neck-and-neck — Conway had a 3-point lead in the final RealClearPolitics average — but Bevin won by a comfortable, 9-point margin.

Read more at https://www.politico.com/story/2017/03/democrats-trump-polling-236560