Democratic rivals Hillary Clinton and Barack Obama have clashed over remarks by Mrs Clinton on civil rights that she says were distorted by the Obama camp.
Black leaders criticised her for saying Martin Luther King's dream of equality came true when President Lyndon Johnson signed the Civil Rights Act in 1964.
On Sunday she said Obama supporters had misrepresented this as a slur on King. Mr Obama denied accusing her of racism.
The black vote will be crucial in the South Carolina primary on 26 January.
It will be the first southern state to vote in the primary season.
"Dr King didn't just give speeches. He marched. He organised"
Mr Obama and Mrs Clinton are leading the race to be the Democratic candidate in November's presidential election.
Both are aiming to build momentum before 22 states nominate their preferred candidate on 5 February, dubbed "Super Tuesday".
'Ludicrous'
The latest clash between the two Democratic frontrunners stem from remarks made by Mrs Clinton during a TV interview last week.
She said Mr "King's dream began to be realised when President Lyndon Johnson passed the Civil Rights Act of 1964" - which could be interpreted as suggesting her experience should mean more to voters than Mr Obama's rhetoric.
KEY DATES AHEAD
In her later clarification on Sunday, she told NBC's Meet The Press talk show that some had misinterpreted as giving Mr Johnson more credit than Mr King.
"Dr King didn't just give speeches. He marched. He organised," she said.
"And he campaigned for political leaders including Lyndon Johnson because he wanted someone in the White House who would act on what he had devoted his life to achieving."
Mrs Clinton went on to say that the Obama campaign was "deliberately distorting" her words.
Mr Obama denied being the source of the adverse comment on her remarks.
"The notion that somehow this is our doing is ludicrous," he told reporters.
Open Republican field
More than half of South Carolina's Democratic voters are African-American.
Analysts say they are likely to vote for the 46-year-old Illinois senator, who aims to become the first black US president.
Mr Obama finished first in the Iowa caucuses in early January, but Mrs Clinton defied pollsters with a convincing victory in New Hampshire.
Meanwhile, Republican contenders are focusing on Tuesday's Michigan primary.
John Mc Cain, who won the Republican race in New Hampshire. said his tenure as head of the Senate Commerce Committee qualified him to handle the economy.
Former New York Mayor Rudy Giuliani - who has not actively campaigned so far - denied that his campaign was running out of money and told Fox News he was skipping Michigan to focus on the Florida race on 29 January.
The Republican race is open - with former Massachusetts Governor Mitt Romney, ex-Arkansas governor Mike Huckabee and former Senator Fred Thompson also regarded as strong candidates.
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