The battle between Bernie Sanders and Hillary Clinton to secure the support of black voters stepped up on Thursday after an important group of black politicians endorsed the former secretary of state.

The Congressional Black Caucus said late on Wednesday that it would support Ms Clinton’s campaign and provide her with an important boost as she prepares for the South Carolina primary, a vote where African American voters will play a crucial role. Meanwhile, Mr Sanders announced he had won the backing of civil rights legend and singer Harry Belafonte.

The caucus said it will will send up to a dozen members to South Carolina to campaign for Ms Clinton over the weekend in advance of the state’s primary election on February 27.

“When we needed someone to come and rally the Democrats and the Congressional Black Caucus, Hillary Clinton has been there,” said Congressman Gregory Meeks, announcing the endorsement on Thursday morning.

“On the issues that are important to our constituents, Hillary Clinton has been there.”

Earlier, another caucus member, James Clyburn, an influential African American politician, told CNN that Ms Clinton had made significant contributions to issues considered particularly important to black Americans, including universal access to health care and children’s issues.

“People keep forgetting that to get to the universal access to health care was a long road,” he said. “People get so caught up in the emotions of a campaign that sometimes they forget that history.”

Ahead of the South Carolina vote - a major test of whether Bernie Sanders can appeal to more diverse populations - the Vermont senator has been investing resources in the state and trying to build up his ground organisation.

On Wednesday, a day after he relished a huge win over Ms Clinton in New Hampshire, he met with the activist and preacher Al Sharpton. They had breakfast at Syvlia’s restaurant in New York’s Harlem, the same venue where Mr Sharpton met with Barack Obama in 2008.

Mr Sharpton said he was not yet ready to make an endorsement for either candidate, but he said that Ms Clinton had to earn African American support.

“And I think she knows it,” he told MSNBC, after his meeting with Mr Sanders.

“You can’t go to South Carolina and not deal with the Walter Scott case, not deal with gun control and the ramifications of the Charleston Nine,” he added, referring to a highly publicised case of an unarmed black man being shot and killed by a police officer, and the attack on a black church last June that left nine people dead.

On Thursday, Mr Sanders also drew support from civil rights icon and entertainer Mr Belafonte.

“I would suggest to those of you who have not yet made up your minds, or maybe even some of you who have made up your minds, to maybe consider and reconsider what it is that Bernie Sanders offers,” said the 88-year-old Mr Belafonte said in a video endorsing Sanders.

“He offers us a chance to declare unequivocally that there is a group of citizens who have a deep caring for where are nation goes and what it does in the process of going.”