If you were asked to assess African-American voting power, you might think about the long history of disenfranchisement that still lingers today, in troubles in Ferguson, Mo.; in efforts to roll back the Voting Rights Act; or in recent moves to limit voting access around the country.

So it can be easy to miss something right in front of our faces: We’re living in an era when blacks have essentially played kingmaker in the most important elections in the nation (thus also the world). And it’s happening again, as Bernie Sanders and Hillary Clinton battle to win the South Carolina primary on Feb. 27.

Mr. Sanders’s first move after his resounding victory in New Hampshire last week was to travel to meet Al Sharpton at Sylvia’s Restaurant in Harlem the next morning. He’s under no illusion: If he can make inroads with black voters and other nonwhite voters in the remaining primaries, he’ll have a chance against Mrs. Clinton. If he can’t make those gains, he won’t, as my colleague Nate Cohn has calculated. Recent history makes it clear:

■ In the 2008 Democratic primaries, Mr. Obama could not have won without unified black support. Mrs. Clinton was the clear favorite. In late 2007, a Pew poll showed her ahead of Mr. Obama by 14 points in South Carolina.

But after Mr. Obama won Iowa, black voters swung behind him, and he defeated her in South Carolina by 29 points. According to exit polls, blacks made up 55 percent of the South Carolina Democratic primary electorate, and Mr. Obama won 78 percent of that vote to Mrs. Clinton’s 19 percent. He went on to roll in states with large black populations, and he needed every bit of that help, barely holding off Mrs. Clinton for the nomination.