I blogged previously on how Democrats should view and oppose Trump. To extend my thoughts, because Trump has few or no principles, he can be unusually flexible (can Teflon be flexible?). Similarly his opponents must be flexible, meaning they should avoid confirmation bias. (See this New Yorker post on this and other ideas relevant to the Trump era.)
We shouldn't believe or argue that Trump is fascist, authoritarian, racist or inept. I guarantee for every ten examples we can point to over the next four years showing those qualities he will have a few counter examples. Our best bet is to attack him as inconsistent, unprincipled, hypocritical showman, of whom the American people will become tired and disillusioned and be willing for a return to Democratic sanity and steadiness in 2020.
The bottom line on Trump is he lost the popular vote by 2.8+ million and won the electoral vote with a lot of luck and a very unlucky opponent. And demographic trends are still against the Republicans. So a competent candidate in 2020 without 40 years of baggage should be favored, even against an incumbent president, assuming Trump will have as rocky a tenure as we Democrats have to believe he will have.