Why do people think Republicans will lose big in mid-terms?

     So it has been some time, but I figured I'd maybe start posting here again. Will have to see how it goes. For this particular post though, I am just wondering about the current claims that the Democrats are going to "ride a wave" into the 2018 mid-terms and win big. My question is, based on what? Yes, Donald Trump has lower poll numbers, but he had those before the 2016 election as well. So if the polls were inaccurate then, than why should they be considered accurate now?

     Public disapproval with Trump's policies? Trump is governing exactly as he said he would. He ran on a total rejection of all the political correctness that has been shoved down the throat of the public for years now, and won. Then he has governed (or tried to govern) based on that and the public is displeased and will thus vote in Democrats in big numbers of 2018? I think people may be greatly over-estimating the Democrats' chances, but we will see.