Biden is said to have picked Jaime Harrison as the next D.N.C. chairman.

President-elect Joseph R. Biden Jr. plans to name Jaime Harrison to lead the Democratic National Committee, according to two people with knowledge of the selection.

A former chairman of the South Carolina Democratic Party, Mr. Harrison became a national political star last year as he shattered fund-raising records in his race against Senator Lindsey Graham. While Mr. Harrison lost in November, drawing 44 percent of the vote to Mr. Graham’s 55 percent, he developed a broad bench of support across the party.

He is also well-known to staff and members of the D.N.C., a result of his work heading the South Carolina state party and a failed bid to become chairman of the committee in 2017. (Tom Perez, the outgoing D.N.C. chair, won that race but has opted against running for a second term.) Mr. Harrison has been championed by Representative Jim Clyburn of South Carolina, an influential Biden ally who helped the president-elect win the primary race in Mr. Clyburn’s home state.

His selection, on the heels of Democrats’ victories in Arizona and Georgia, reflects Mr. Biden’s longstanding desire for Democrats to compete in once-red states, a recognition that the party will never sustain an enduring congressional majority without making inroads across the Sun Belt.

Incoming presidents traditionally take control of the party committees, installing their own chair and staffers. Former President Barack Obama chose to try to establish his own political operation outside of the committee, a decision that many D.N.C. members say damaged state parties and led to years of dysfunction at the national level.

Far more of a party institutionalist, Mr. Biden has promised to rebuild state parties and deepen investments in the committee. Following the tradition of committee members deferring to the president’s pick, Mr. Harrison is not expected to face a challenger for the position.

Democrats are navigating a deeply uncertain political landscape. Even before the attack on the U.S. Capitol scrambled American politics, Democrats anticipated difficult House and Senate midterm races in 2022 — the president’s party usually loses seats in such elections — and the possibility that Mr. Biden, who will become the oldest president in U.S. history on Wednesday, may decide not to run for a second term.