Their primary focus should be retaining voters, by broadening the range of opinions which are acceptable within the party.
They are a decade down a purity spiral, which has resulted in the range of acceptable opinions within the party shrinking considerably, and the shunning of many individuals unnecessarily, who either stop voting altogether or find company on the right.
I gave the example above of how the republican party is able to accomodate a significant number of both pro-life and pro-choice members. The Democrats will similarly need to learn to expand their umbrella as well. Perhaps not with abortion rights, but maybe by shedding some of their zero-sum economic thinking, or race-centric thinking.
If they can fix this, they will grow, because their biggest source of new members is young adults becoming eligible to vote, not people they pull away from the right. The Democrats just need to stop churning so many people away.