The majority of the public aren't interested in politics and couldn't care less outside a major presidential election. The Mid-terms are more a show of whose base is more active at the time, which historically, the sitting president's party tends to lose, since they become complacent with victory. The Dems were expected to win the Mid terms based on this trend alone.
The fact that the Mid-terms were as close as they were, in spite of heavy social media censorship of Republicans and MSM propaganda in opposition--not allowing for an even playing field for people to become informed of policies--is hardly reflective of such, and not a good sign for the Democrats in 2020... where they have to take on Trump himself, and the majority of America will be paying attention.
If you think it's policy that has taken them this far, you may be in for a rude awakening. Since the loss of 2016, the Dems have doubled down and gone even further left, deviating away from centrists/moderates. While Trump is evidently appealing to more of the political spectrum, as shown through his relatively high approval rating.