Democratic voters in South Carolina early on Saturday began casting their votes on Hillary Clinton and Barack Obama in a presidential primary scarred by bad blood between the bitter rivals.
The "first in the south" nominating contest is the culmination of a fiercely divisive state campaign, which has at times taken the party into dangerous racial ground, and including fiery interventions by ex-president Bill Clinton.
It is also the final contested Democratic nominating clash before February 5, "Super Tuesday" when nearly two dozen states hold contests on a night that could be decisive for this year's presidential candidates.
Polls opened at 7:00 am (1200 GMT) and were set to close 12 hours later.
Expectations are highest for Obama, who desperately needs a victory after Clinton scooped the Nevada caucuses and New Hampshire primary, despite his win in the leadoff Iowa caucuses on January 3.
As he strives to become the first black president in US history, Obama has built a powerbase among African American voters, who form about half of the Democratic electorate here, and should be enough to take him to victory.
"For Obama, a win in South Carolina is the equivalent of holding serve," said University of New Hampshire political scientist Dante Scala, predicting the Illinois senator would not get appreciable momentum from an expected victory.
Even before the vote, Clinton was looking ahead to February 5, with trips this week to populous states like New York, New Jersey and California, leaving her husband behind to wage a daily back-and-forth with Obama.
Both top candidates, and third ranked John Edwards, who won the primary in 2004 but has failed to catch fire this time, sprinted to the finish line Friday with a string of statewide rallies and events.
"We are in a very close race here ... I have no idea what is going to happen tomorrow," Clinton said.
Trying to stem sliding support among African Americans, Clinton appeared with several prominent community leaders, who pleaded with voters not to pick Obama just because he is black.
"It may take a very, very bold step to walk into that voting booth focusing on our community's interests, rather than simply acting on emotion," said Stacey Jones, dean of largely black Benedict College in South Carolina.
Obama drew 3,000 people, many students, to hear his spellbinding rhetoric at Clemson University, in Greenville.
"Change in America had always started with young people," Obama told the crowd. "This is your moment, this is your time."
Earlier, Clinton tried to ease tempers after a week of accusations of truth twisting, and claims by both camps that the other was playing the race card.
"That all needs to just calm down, and everybody needs to take a deep breath" she said on CBS.
Obama's camp however accuses Bill Clinton of fanning the flames, and all but accused the former president of lying about the record of his wife's rival.
An MSNBC/McClatchy poll Friday showed Obama leading his rival by 38 per cent to 30 per cent in South Carolina, based largely on staunch backing from African Americans. Former senator John Edwards was third on 19 per cent.
But Obama's standing among whites in the southern state had plunged 10 per cent in just one week, despite his efforts to portray himself as someone with cross-racial appeal.
A Wall Street Journal/ NBC poll also had troubling signs for Obama, showing Clinton now leads the Democratic race nationally among white Americans 53 per cent to 24 per cent, compared to a 40-23 per cent margin last month.
Obama led Clinton, 63 per cent to 23 per cent among the minority African American community nationwide.
Meanwhile, Ohio Congressman Dennis Kucinich, a staunch opponent of the war in Iraq, officially ended his White House bid on Friday, The Cleveland Plain Dealer, his hometown newspaper, reported.
"I'm directing my energies to be reelected to the Congress of the United States," he is quoted as telling supporters.