Regardless of which strategy they deploy in the next eight years, I think the democrats have some math issues to contend with.
For the last ten years and five election cycles, they have been happy to sit on their "big blue wall" for presidential elections and lose the state count 27-23 or somesuch because the 23 had enough EVs to stop the GOP from entering the white house. The problem with that strategy is that it hollows you out at the state and legislative level. You give the GOP the majority in the senate and the house, you give them more and more ground in state houses and governor's mansions.
The thing I see is that the left believes in governing from the top down (always has) and is thus primarily focused on the presidency. They've become like the 2003 Mariners - no prospects, a bunch of veterans running out the clock to retirement who are year to year as to their viability. They don't have a bench presently either...it's all about who their starters are. They have to grow a new generation of leaders - and it's hard to do that if the other party controls most of the local, state, and federal positions beneath the Presidency. It's been Obama's party...they've spent no time on developing his natural replacement.
The energetic faces of the party are Howard Dean and Bernie Sanders...two guys who represent a bygone generation and are on their way out.
Meanwhile, as much as the left doesn't want to see it, the GOP has an enormous farm system, a generation of diverse, young, exciting politicians at every level of government, and believes in governing from the bottom up.
That isn't to say that the left can't recover...they can and will focus, now, on developing young talent...but they're in a rebuilding year and it requires a five-year plan.
And, unfortunately for them, they picked a bad time to be rebuilding, because the GOP is going to have 3-4 supreme court nominees coming up and by the time they get back into contention, the umpires will be stacked 6-3 or 7-2 against them.
In any event, the party that holds the presidency usually loses ground during their tenure, especially if they run for 8 years. But the losses the democrats have taken under Obama are matched in modern history only be the collapse of 1930 (GOP losses due to Hoover's general ineptitude), and the late collapse of the democrat party in the 1940s (FDR/Truman governed for too long and people basically told them to stop).
So right now, the left is hoping Trump is as bad as they think he is (and I fear he is) and will scare people enough that they run back their way...we shall see.