I feel like Democrat are going to need to ask themselves what worked so well in 2018 that didn't work as well this year. I understand it was a midterm and not a presidential race, but it seems Democrats made much more progress then.
My take, candidates spent a lot of time talking more about healthcare than just talking about how bad Trump is. It's a winning issue for them and should have been hammered relentlessly especially with the current pandemic and Trump's lack of response to not having his own healthcare plan.