"We were getting some indications that might be the case not only from the poll numbers," she said.
"But also from the fact that we were getting wind from the Obama camp he was planning a victory party while we heard that Hillary Rodham Clinton had headed on to Nashville, Tennessee."
Clinton left South Carolina before the polls closed and headed to Tennessee, which votes on February 5, for a town hall meeting.
The South Carolina vote marked a second victory for the Illinois senator and evened the score with Clinton, who has also won two key state primaries.
"Tonight, the cynics who believed that what began in the snows of Iowa was just an illusion were told a different story by the good people of South Carolina," Obama said in his victory speech.
"In nine short days, nearly half the nation will have the chance to join us in saying that we are tired of business-as-usual in Washington, we are hungry for change, and we are ready to believe again," Obama told a cheering crowd.
"There are real differences between the candidates. We are looking for more than just a change of party in the White House. We are looking to fundamentally change the status quo in Washington."
'The race card'
In the lead up to the primary, each side accused the other of playing "the race card".
Bill Clinton, the former president and husband to Hillary Clinton, was quoted as saying candidates were "getting votes, to be sure, because of their race or gender".
"That's why people tell me Hillary doesn't have a chance of winning here," Clinton said, suggesting that blacks would not support a white alternative to Obama.
Clinton campaign strategists denied any intentional effort to stir the racial debate, but said they believed the fallout has branded Obama as "the black candidate".
Four out of five black voters supported Obama, who would be the first black US president, according to polling place interviews conducted on behalf of the Associated Press.
Clinton and Edwards each won roughly 40 per cent of the white vote, with about 25 per cent going to Obama, the poll showed.
"The question will be whether he [Obama] can now carry this victory into other states where there is not a majority of African-American voters," Kathleen Kendall, from the University of Maryland, told Al Jazeera.
Regardless of the focus on race, the Associated Press poll showed that half of the Democratic voters said the economy was the most important issue, while about one quarter picked health care and only one in five said it was the war in Iraq.
The victory gives the Illinois senator a boost ahead of "Super Tuesday", when nearly half the states will pick Republican and Democratic candidates for the November election.