>>The democrats believed the false polls they created.<<
They were spun on purpose. The tendency for the last 20 years or so has been to goose polls to the left to make Conservative voters despondent and not show up. It worked marvelously in 2008 ans 2012.
In 2016 people in flyover country were galvanized and motivated. They didn’t care what the polls said: they wanted to Make America Great Again.
And did.
No kidding. Other evidence might be to look at why they have lost 1000 seats in 8 years - if they were honest with themselves. Try doing honest polls, with honest data, and doing honest analysis and then they might get a clue (I do not expect this to happen, I expect nothing to change).