To show this, I ran a simple, illustrative scenario of a 2020 primary between former Massachusetts Gov. Deval Patrick and Sens. Kirsten Gillibrand and Sherrod Brown. I assumed that each state would have the same number of delegates that it did in 2016 (an incorrect assumption but, again, this is illustrative). In every state where blacks made up more than 20 percent of the overall population, I gave Patrick 40 percent of Hillary Clinton’s delegate haul (Clinton’s coalition in those states was often majority black). In every other state, he got 20 percent of Clinton’s share of the delegates and 15 percent of Bernie Sanders’s. In 2020, Patrick might perform significantly better or worse than this (if he were to run), but the point is that he might take some votes from a more center-left candidate like Clinton (especially in heavily black states) and a bit less from a died-in-the-wool progressive like Sanders. I gave Gillibrand any Clinton delegates that didn’t go to Patrick, and Brown, a progressive, got the remainder of the Sanders delegates. Under this scenario, Gillibrand, Brown and Patrick won 42 percent, 39 percent and 19 percent of the pledged delegates (those won in primaries and caucuses), respectively.
In other words, no candidate would have majority in an explicitly majoritarian system.
In past Democratic primaries, superdelegates might have swooped in to solve the problem. For example, in 2016 Clinton won a majority of pledged delegates but failed to get a majority of all delegates, so the superdelegates decided to push her over the top well before the convention. And in this fictional scenario, they might coordinate and give Gillibrand (the plurality winner) the nomination.
But there’s no guarantee that would happen in 2020.