How each 2020 Democratic presidential candidate could win

Now that New York Mayor Bill de Blasio has entered the 2020 Democratic primary, we think the field is set. (Okay, Georgia politician Stacey Abrams could decide as late as the fall, but that’s the only question mark we know about.)

That means at least two dozen people are seriously competing in the primary. With each new entrant, I’ve been asking myself: What would it take for this person to win the nomination? In some cases (with, say, Biden or Sanders), it’s pretty clear. That question becomes much more difficult,...