For years, black Americans have cast about one out of four votes in Democratic primaries. In 2016, they cast 71% of Democratic primary votes in Mississippi; 61% in South Carolina; 54% in Alabama; 51% in Georgia; 46% in Maryland; 32% in North Carolina and Tennessee; and 20%-28% in Arkansas, Florida, Illinois, Virginia, New York, Michigan, Missouri, and Ohio.
Near-unanimous black support helped nominate and elect the last three Democratic presidents — Jimmy Carter, Bill Clinton and Barack Obama. Near-unanimous, because black primary voters have tended to vote with solidly for one candidate over another, even against alternatives with serious claims on their support.
Such solidarity in voting makes sense for people identifying as part of a distinct group suffering from discrimination. For years, political reporters have listened as black preachers, avoiding outright endorsements, called for “unity.” Their congregations understood what they meant.
There are signs that black voters may not be behaving as monolithically as they used to. Exit polls in the 2016 Democratic presidential primaries showed them favoring Hillary Clinton pretty unanimously in the South, giving Bernie Sanders only 6%-19% in former Confederate states. But in New York, Pennsylvania, five Great Lakes states, and Missouri, Sanders got between 26% and 32%; he carried three of these and came within 2 points of carrying two more.