Dems must not run on something silly like "return to normalcy"' that's some Hillary empty campaign.
They have to go out, find people directly affected by each and every move the GOP makes, and tell them how it's impacting them. Do that everywhere, all the time, and on a national scale. The biggest weakness they'll have is having an actual presidential candidate, but in 2018 they could do really well if they stuck to this strategy, and if they do then it makes it easier in 2020.
But really, there is no super move to use, they have to go back to doing politics as usual, which is bitching about what the other party is doing and why it's all awful. That actually works. Forget the crazy people who will support Trump forever, 2016 was lost because people felt they could afford something different and Hillary couldn't sell a next-step of four to eight more years of Democrats. Completely different situation now, it's back to doing what always works in politics, not looking for the next Buddha's reincarnation or the road to the promised land. Leave the preachy stuff out, it was good for Obama, he's gone, focus on the impact of the GOP's policies, and forget the clown.
This isn't even difficult to do, the Dems have the easiest road to take ahead of them. GOP has a crazy clown as presient, and a majority, they can be blamed for everything. Any other opposition party in the world would win this easily.