In the past, white Southern moderate politicians (famously Jimmy Carter and Bill Clinton) showed themselves capable of putting together coalitions that included noncollege-educated white voters in the South and border states along with African-Americans from a similar religio-cultural background. That type of politician barely even exists today. African-American and Latino voters are beginning to gravitate toward presidential candidacies from their own communities, which may be more self-consciously progressive.
More subtly, we may be entering an era in which instead of Democratic progressives casting their lots with competing moderates of somewhat different dispositions (you saw a lot of that in 2008 when lefty activists split between Barack Obama and John Edwards), Democratic moderates split between competing progressives. I am impressed at how many of my own moderate Democratic acquaintances are attracted to Elizabeth Warren as a counter-weight to Bernie Sanders.
But all these trends cut across the currently immovable presence of the one 2020 candidate who will almost automatically appeal to Democratic moderates: Joe Biden.