Mr. Biden, who lost badly in Iowa and New Hampshire but improved in last weekend’s Nevada caucuses, appeared on track to win a majority of black Democrats in South Carolina, with voters citing his familiarity with their concerns and his potential to appeal to moderate Republicans in a general election. But the final margins may reveal fault lines around gender, age, regions and viewpoints.

Other Democratic hopefuls from across the ideological spectrum — liberals like Mr. Sanders and Senator Elizabeth Warren, as well as the billionaire Tom Steyer and moderates like Mr. Buttigieg — were aiming to demonstrate that African-Americans are not a monolithic voting bloc.

Many black Democrats made that point over the last year, as some favored white candidates like Mr. Biden over black contenders like Senators Kamala Harris and Cory Booker, and others embraced a wide variety of political and ideological positions.

Mr. Biden led in South Carolina polls all year, but Mr. Sanders and Mr. Steyer gained ground recently, and Ms. Warren and Mr. Buttigieg were seeking to show that they have black support in the primary.

“I don’t know that anybody’s going to walk away with it. And I think there might be some surprises,” said the Rev. Joseph Darby, a pastor who leads Nichols Chapel A.M.E. Church in Charleston. “But I don’t pay attention to the polls because they never call me.”

At a ministers’ breakfast in North Charleston on Wednesday hosted by National Action Network, the civil rights organization led by the Rev. Al Sharpton, several voters echoed his point — stating that the question of “what Democratic candidate is most electable” in the primary could not be settled until black voters throughout the South weighed in on the nominee.