Bernie Sanders's presidential campaign has always known that it needs to play catch-up with black voters if it's going to win the Democratic primary. South Carolina was Sanders's first big chance to show he could make inroads with this group, and it looks like he completely and utterly failed.

Clinton winning black voters 84-16, bigger margin than Obama in 2008, when he won 78-19 — Martin Gelin (@M_Gelin) February 28, 2016

That is, needless to say, very bad news for Sanders. And if he can't change those numbers, it's going to be a huge problem for him throughout the South as well as in several Northern states like Maryland, Delaware, New York, Illinois, and New Jersey.

But beyond the primary math, it raises a kind of conceptual issue about Sanders's political theory: Given the centrality of African-American voters to the Democratic Party, it's hard to envision a left-wing political revolution taking place in the United States that they have so little enthusiasm for.