They should focus first and foremost on improving the economic condition of the average American. The low income, as well as the middle class slipping into poverty. Worker's rights is a major part of that, but only one part of it. Watching the prices of basic necessities like housing, food, and healthcare while billionaires and corporations are making record profits is bound to piss off the people.
That said, Trump certainly isn't going to make any of that better. In fact, it'll all get much worse, but on the slim chance democrats actually try to win voters back vs just counting on America to come crawling back to save the US from the four year shit show we've just started and if our new dictator allows us to have fair elections in the future, I think you've got the right idea for where they can start.