HOPKINS, S.C. — Whether a simple question from an elderly church parishioner persuaded Representative James Clyburn to make his seismic endorsement of Joe Biden before the South Carolina primary will probably be known only by Mr. Clyburn himself.

But if the endorsement and the ballots of black voters in South Carolina and across the South transformed the presidential race, it would be hard to find a more appropriate co-star in the drama than Jannie Jones, a 76-year-old church usher whose great-grandmother came to South Carolina aboard a slave ship.

A week after the South Carolina vote, she was still elated by her Feb. 21 encounter with Mr. Clyburn, whom she had never met before, and was astonished to have played a role in what might have been the most important moment in the Democratic race.

“I’m still sitting here and having a hard time believing it,” Ms. Jones recalled in the living room of her tidy home a couple of miles from the church. She added, “Joe was nearly dead, and now he’s come back to life.”