James Clyburn's support was intensely sought by all the main candidates after his original choice, Dick Gephardt, dropped out of the race. The endorsement is vital for Mr Kerry, who is desperate to prove he is both a viable candidate in the South and can garner black support, which could account for 50% of the electorate.
"After suffering three straight years of job losses, South Carolina can't afford four more years of President Bush," said Mr Clyburn. "John Kerry has a vision to get South Carolinians back to work and give everyone the opportunity to realise their own American dream."
South Carolina lost 18,900 manufacturing jobs in the past year, leaving the state with an unemployment rate above the national average, and a large number of disaffected blue collar workers who previously voted Republican.
Mr Clyburn's endorsement will come as a blow to John Edwards, who was born in the state. It could also prove fatal for Wesley Clark, who was hoping his Southern background would give him some leverage.
With polls in South Carolina giving Mr Edwards a narrow lead over Mr Kerry, the race is set to pitch Mr Kerry's momentum against Mr Edwards' local credentials with the wild card of Al Sharpton's appeal to black voters leaving the outcome uncertain. "This is a two-man race," said former South Carolina Democratic party chairman, Dick Harpootlian. "It's Kerry and Edwards."
Meanwhile, Howard Dean's campaign showed signs of strain after it became clear he was only confident of having enough money to compete until next week. Mr Dean has told his staff they will have to skip pay for two weeks and organisers in Wisconsin were informed they would not be paid for a month.
Mr Dean's campaign chief, Joe Trippi, resigned after Mr Dean appointed a friend of former vice president Al Gore to try to restart his campaign. Mr Dean is the only candidate who is not airing television ads in any of the seven states at stake on Tuesday.
While Mr Kerry remains a favourite at this stage, picking up key endorsements across the map, none of the states due to be contested on February 3 are sewn up. Along with South Carolina, Arizona, Oklahoma, Delaware, North Dakota and New Mexico are all wide open. Polls show Mr Kerry is ahead in Missouri, but Mr Clark is leading in Oklahoma, while Joseph Lieberman, believed to be all but out of the race after his fifth-place showing in New Hampshire, received the endorsement of the newspaper the Arizona Republic.
All but two of the states up for grabs on Tuesday voted Republican at the last election. With a higher proportion of military veterans and the first Southern state in play, the candidates are pitching their message to the centre, talking less about gay marriage and more about jobs and health care.
With Mr Dean ahead in none of the forthcoming contests, his campaign is considering a shift in strategy by looking ahead to the primaries in Michigan and Washington state on February 7 and Wisconsin on February 17. But if Mr Kerry emerges triumphant on Tuesday it may be too late for Mr Dean by then.