In 2016 Democrats ran against Donald Trump the person. In 2020 they should/will run against President Donald Trump. It'll be a referendum on Trump as President, not as much about Trump as a person.
I keep hearing that turnout was low and that since Donald Trump got less votes than Mitt Romney he won because Democrats stayed home. Turnout wasn't low. As of now, turnout is down 3.5 million from 2012, but California is down 4.1 million. Washington state is down 933k compared to 2012. These two states have always counted a large number of ballots after election day. If you exclude California and Washington turnout is up 1.5 million. Over the next couple of weeks these two states will count all their ballots and we'll surpass the 2012 count. As of now, Texas and Florida are up 11%, Oklahoma up 9%, and Nevada up 8%. These states will also add ballots. Yesterday an additional 1.4 million ballots were added to the total and some states haven't begun counting their late and provisional ballots.