Image copyright AFP/Getty Images Image caption Mrs Clinton has an advantage going into South Carolina - for now

As the presidential race heats up and attention turns to Nevada and South Carolina, two states with more diverse populations than Iowa and New Hampshire set to vote next, there has been lots of talk about the importance of the "black vote".

But what is the "black vote?" Black voters were hugely important for Barack Obama's win in 2008, but the vote is not a monolith.

Support for Hillary Clinton and Bernie Sanders is splitting among black groups of voters. We take a look at some of the factors in this nuanced issue on the heels of the battles to come.

Congressional Black Caucus PAC support - and division

Image copyright AP Image caption The Congressional Black Caucus Political Action Committee have stated their support for Mrs Clinton

The Congressional Black Caucus Political Action Committee - the political arm of the Congressional Black Caucus - came out in support of Hillary Clinton on Thursday.

"She has been, her whole career, an individual fighting for issues important to the African-American community," said Congressman Greg Meeks, chairman of the group. "The person that has partnered with the CBC over the years is Hillary Clinton."

Another member of the Congressional Black Caucus, however, tweeted that the group's endorsement does not represent the CBC as a whole.

"Cong'l Black Caucus (CBC) has NOT endorsed in presidential. Separate CBCPAC endorsed withOUT input from CBC membership, including me," tweeted Keith Ellison, a representative from Minnesota who has endorsed Bernie Sanders.

During a press conference, other members of the PAC derided Mr Sanders for not supporting the Brady gun control bill, which strengthened background checks, and for voting in 2005 for a bill that gave immunity to gun manufacturers (he recently co-sponsored a bill that would end the practice). It's too easy for children in African-American communities to access guns, they argued.

"Bernie Sanders has not just been missing in action, he's been on the wrong side," on guns, said Hakeem Jeffries, a representative from New York. PAC members said they would be heading to South Carolina to urge voters to go with Mrs Clinton in the primary.

"No-one is better prepared to be president than Hillary Clinton," said John Lewis, a representative from Georgia and former civil rights leader. "We need her leadership, dedication and vision more than ever before. We must get out our vote like we never, ever voted before."

On the pro-Bernie side

Image copyright EPA Image caption Mr Sanders had breakfast with Reverend Al Sharpton, a known civil rights activist, the day after the New Hampshire primary

Black legal scholar Michelle Alexander argues in The Nation magazine that Hillary and Bill Clinton "decimated" black America during Mr Clinton's presidency by supporting laws that broke down their communities.

Her article, "Why Hillary Clinton Doesn't Deserve the Black Vote," argues that in the election, there's such a thing as a lesser evil, and Mrs Clinton is not that.

"When politicians start telling you that it is 'unrealistic' to support candidates who want to build a movement for greater equality, fair wages, universal healthcare, and an end to corporate control of our political system, it's probably best to leave the room," writes Ms Alexander.

Black writer Ta-Nehisi Coates of the Atlantic, author of "The Case for Reparations," has come out in support of Bernie Sanders, despite Mr Sanders having said he is opposed to reparations for black people over slavery in America because it is too "divisive" of an issue.

"One can say Senator Sanders should have more explicit antiracist policy within his racial justice platform, not just more general stuff, and still cast a vote for Senator Sanders and still feel that Senator Sanders is the best option that we have in the race," he told Democracy Now.

Ben Jealous, former president of the National Association for the Advancement of Coloured People, has also come out in support of Mr Sanders.

He told MSNBC's Rachel Maddow this week that he respects Bernie Sanders for holding onto idealism.

"My theory is…game recognises game. You send your idealist, we send our idealist, and a compromise will be a real compromise."

'Black pragmatism'

New York Times columnist Charles Blow criticises some Bernie Sanders supporters for "Bernie-splaining" to black voters - arguing that they do not know Mr Sanders represents their best interests - in a column this week.

These arguments are condescending, offensive and paternalistic, Mr Blow writes.

"Black folks are trying to keep their feet planted in reality and choose from among politicians who have historically promised much and delivered little," he writes.

"History and experience have burned into the black American psyche a sort of functional pragmatism that will be hard to erase. It is a coping mechanism, a survival mechanism, and its existence doesn't depend on others' understanding or approval. However, that pragmatism could work against the idealism of a candidate like Sanders."

To some black voters, Mr Sanders' campaign promises simply sound too good to be true, and Mrs Clinton's record and reputation is more rooted in reality and what can actually get done in a Republican-controlled Congress.

Onto South Carolina

Image copyright Getty Images Image caption Actress Vivica Fox has camaigned for Mrs Clinton in South Carolina

A recent NBC News/Wall Street Journal/Marist poll in South Carolina shows that Mrs Clinton has a 74% to 17% advantage among blacks in the state.

In 2008, exit polls showed that 55% of South Carolina's democratic electorate was black.

So Mrs Clinton certainly has an advantage going into South Carolina. However, that doesn't mean South Carolinians won't change their minds come election day, or that young people and university students won't flock to Mr Sanders like they did in New Hampshire and tip the equation.

Some in South Carolina are asking questions about the former secretary of state's track record. "I did my background research on what Hillary has really done for the black community," student Taylor Honore told NPR, "and it kind of concerned me".