A few thoughts:
Trump barely won against Clinton primarily as a protest candidate against politics as usual with the understanding that he wouldn’t win so voters could safely vote for him or stay home. That attitude changed November 7th 2016. I don’t think there is a Liberal alive in the US who isn’t motivated to vote against Trump. All thing being equal, there is no reason that A Democratic candidate shouldn’t win handily.
The key being “all things being equal”. With a thoroughly partisan Department of justice, a Federal Election Commission that is gridlocked to irrelevancy, and a Republican Party that sees Democracy as a hindrance to their political goals, we should expect that there will be wide spread attempts to do use every dirty trick in the book to prevent Democrats from voting, or from counting when they do vote. These efforts need to be identified and countered as quickly as possible, and also publicized as evidence of the anti-democratic nature of the Republican party.
Which brings us to my third point. I am going to go counter to conventional wisdom and say that after the primaries Democrats should not concentrate on a clear positive message for what policies they will bring to the country. Policies are by their nature complicated and imperfect. Any realistic policy will include pluses and minuses. Republicans will find those minuses and focus all attention on what they are. So that instead of being the anti-global warming candidate, the Dem will be the the pro high gas price candidate. You can try to argue back and forth about which is true and which is a bigger problem but it ends up being at best a draw. 2016 showed that empty promises with no details are just as good if not better than complicated policies, but that the real vote getter is fear. People vote with their amygdala, not their cerebral cortex.
I would recommend concentrating on the president’s corruption, lies, contempt for any checks and balances, and autocratic tendencies. Make it clear that what Trump has been normalizing is anything but normal and that if we continue down this road, we may go past the point of no return. It is easy to cobble together the statements that Trump as made to indicate that he wouldn’t be satisfied with just two terms. We want to leave the impression that the 2020 election is all that stands between us and the world of the man in the high castle.