Numbers to watch for when results come in

South Carolina polls close at 7 p.m. and usually they report their results quite quickly. In the 2016 Democratic primary, 86 percent of the vote was counted by 9 p.m.

In the event of an early call, here are two numbers to keep in mind:

30 The delegate margin Mr. Biden needs to take the delegate lead over Bernie Sanders.

The delegate margin Mr. Biden needs to take the delegate lead over Bernie Sanders. 86,049The margin of victory in the statewide popular vote that Mr. Biden would need to take the lead in the cumulative popular vote over Mr. Sanders.

South Carolina political geography

Mr. Biden is considered the favorite because of his strength among black voters, who represent a majority of Democratic voters in the state. On average, Mr. Biden leads by 15 points in the most recent polls, and he could win by much more than that in rural, midland and low country counties where nearly all Democratic voters are African-American.

Mr. Sanders might fare better in areas where white voters make up a larger share of the electorate, like the populous but Republican upcountry Piedmont region, or even along the coast in a competitive race.