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Benjamin T. Jealous, a prominent black leader and former N.A.A.C.P. president, endorsed Senator Bernie Sanders of Vermont for president on Friday and vowed to increase his support among black voters in South Carolina, a crucial primary state, as well as across the country.

Mr. Jealous, who led the N.A.A.C.P. from 2008 to 2013, said he gave his endorsement because Mr. Sanders opposes the death penalty, will combat institutional racism and has “the best plan for ending mass incarceration and improving community policing.”

As the race for the Democratic presidential nomination moves beyond Iowa and New Hampshire into more diverse primary states, Mr. Sanders must work hard to compete with Hillary Clinton, who has a big lead on Mr. Sanders among black voters, who have known and supported her and former President Bill Clinton for years.

Mr. Jealous said he planned to spread Mr. Sanders’s message and believed the race between Mr. Sanders and Mrs. Clinton was reaching “a real tipping point.”

“Having helped strengthen Obama’s efforts in 2008, I know how things can turn very, very quickly,” Mr. Jealous said. “As somebody who has organized throughout the entire South, I intend to be very engaged and to help build the campaign in the South and quite frankly help to build it in the cities of the Midwest and throughout the country where you see both large black and brown populations.”

Mr. Jealous and Mr. Sanders made the announcement during a conference call on Friday after snow in New Hampshire forced the campaign to cancel a news conference in Exeter, N.H.

In explaining his plan to help Mr. Sanders, Mr. Jealous also praised his past work and his policy ideas.

“Throughout his life he has been a fearless, tireless and trustworthy champion for the right of all our nation’s children to have full and unfettered access to the American dream,” Mr. Jealous said. “Bernie Sanders has the courage to confront the institutionalized racism and bias that stains our nation. Bernie Sanders leads with the sort of freedom-minded conviction that strikes fear in the military industrial complex, the prison industrial complex, and the worst of Wall Street.”

Mr. Sanders stressed that his plans to make public colleges tuition free, to raise the minimum wage and to focus on creating jobs would especially help people of color, who face higher rates of incarceration, and black youth, who face higher rates of unemployment. The senator also said, as he has in the past, that if elected he would work on reforming police departments and federally decriminalizing marijuana possession.

He added that he knows he is not well known in South Carolina, especially among black people, but expects to make “huge progress” in the state. He also said that as soon as New Hampshire votes, he would be spending as much time as possible in Nevada and South Carolina, where the nominating contests come later this month.

“I think, frankly, that we do have the agenda that will make sense to the African-American community both in terms of economics and in terms of criminal justice and other issues,” Mr. Sanders said. “I think our job is to get that agenda out aggressively, and I think Ben Jealous is going to be an enormous help in enabling us to do this not only in South Carolina but across the country.”