I think that's the wrong question.
I think the question is: Who can the Democrats nominate that can unify enough of the political left and center to offset what I see will be the Trump coalition in 2020? I think there are easily enough votes and electoral college votes to defeat Trump, if they will come together to do so.
The Trump coalition that I see consists of die-hard Trump supporters and Republicans who see benefits from his actions as President even if they have to hold their noses to vote for him. Some of those actions are his appointment of judges and probably one or two more Supreme Court Justices, his hands-off approach to the decimation of professionalism in executive agencies and his reduction of the US's role in the world, commensurate with an isolationist approach to the world.
I am not sure I see a Democrat nationally visible at the moment to make that happen. Kamala Harris appears to be very bright and attractive as a candidate, but can she build a national awareness that's sufficient to win the electoral college? Some of those who have expressed interest, such as Julian Castro, Eric Garcetti, Martin O'Malley and Joe Biden all are potentially interesting but have to work very hard to build a profile or to change their existing profile to have a chance, I think. Any of these candidates will have to win over several constituent groups to have a chance to beat the Trump coalition I described above.
Remeber, it's the electoral college that has to be won.