Photo

COLUMBIA, S.C. — Representative James E. Clyburn plans to endorse Hillary Clinton for president on Friday, giving her campaign a powerful seal of approval from one of the most influential Democrats in South Carolina and the leading black politician in the state.

Mr. Clyburn is expected to announce his support for Mrs. Clinton during a news conference at Allen University in Columbia, according to a person close to Mr. Clyburn with knowledge of his plans, who spoke on condition of anonymity.

Mr. Clyburn, who is a member of the Democratic leadership in the House of Representatives and the lone Democrat elected to Congress from South Carolina, has long been a sought-after power broker in presidential primary campaigns. He stayed neutral in the 2008 presidential primary, opening a rift that year between him and the Clintons. It deepened after former President Bill Clinton dismissed Barack Obama’s success in South Carolina as comparable to the Rev. Jesse Jackson’s 20 years before, a remark that offended many African-Americans.

Mr. Clyburn’s support for Mrs. Clinton now, at such a key time in her campaign, is a sign that her and Mr. Clinton’s efforts at reconciling with Mr. Clyburn yielded results.

His endorsement this year comes at a particularly important moment, as Mrs. Clinton makes an all-out push for black voters’ support in her primary battle with Senator Bernie Sanders of Vermont.

Mrs. Clinton has held a strong lead in the polls here, largely because of the strength of her support in South Carolina’s black communities. Mr. Clyburn’s backing can only reinforce that support.

A Bloomberg Politics poll published Thursday found Mrs. Clinton leading Mr. Sanders here by 22 percentage points over all, and by 39 percentage points among black primary voters. The South Carolina Democratic primary, the fourth contest in the 2016 race, will be held Feb. 27.