Rep. Jim Clyburn, D-S.C., made clear that his endorsement was based, at least partly, on Kerry's ability to beat President Bush in the general election.
Clyburn's endorsement could be significant in the February 3 primary, where as many as half the Democrats casting ballots are expected to be African-American.
The state's senior U.S. senator, Ernest "Fritz" Hollings, has already endorsed Kerry.
In accepting Clyburn's endorsement, Kerry hit hard at the Bush administration's economic record, noting that the state's textile industry has suffered extensive job losses.
Kerry promised to crack down on tax breaks for companies that close U.S. plants to move work overseas.
"It's unfair when the government of the United States of America asks taxpayers, through our tax code, to actually subsidize it," he said. "I will close every loophole, every benefit, any reward for any Benedict Arnold company or CEO who decides to take American jobs and money overseas and stick the American people with the bill."