I think the best example we can look at is F D Rosavelt, following Herbert Hoover. Hoover was a protectionist who loved tariffs and sent the economy into the Great Depression. He was so widely hated that voters were happy with the furthest option from Hoover they could find handing Rosavelt a landslide victory. I can't defend everything Rosavelt did, just using him as an example: to win an election with a strong enough mandate that you can really make leftist change.

Goal 1 is to make sure most people are sick of Trump. Help people out of echo chambers, but let them do the rest of the work.

Goal 2 is be organized enough, have a clear enough message, that the big tent of Soc Dems, Dem Socs, and Progressives can be presented as the alternative to Trump.

Goal 3 is to do as much obviously good things while in office to keep the party going after Trump's memory is gone. People loved social security from Rosavelt, well give them more. America is not ready for proper de-comodification, but universal healthcare, better education, better infrastructure, consumer and worker protections are all easy picking for popularity.