Hillary Clinton and Bernie Sanders are facing off in the South Carolina primary, the latest battleground for the Democratic presidential nomination.
Mrs Clinton is seen as the favourite to clinch the state.
But on the eve of the vote, Mr Sanders was endorsed by Robert Reich, a former official in Bill Clinton’s presidential administration.
Both contenders are hoping to build momentum ahead of “Super Tuesday” primaries in 11 states next week.
South Carolina is the Democrats’ fourth nominating contest since the primaries began on 1 February.
“The South Carolina primary is personally important to me because I want to send a strong signal that South Carolina is ready for change, ready for progress, ready to make a difference,” Mrs Clinton, 68, told a rally in the state capital Columbia on Friday.
Eight years ago, she lost the state overwhelmingly to then Senator Barack Obama.
Observers say a victory here would also establish Mrs Clinton as the firm favourite among black voters, a key section of the Democratic electorate, and set her up to garner a big haul of delegates in Super Tuesday contests in the South.
Rival Bernie Sanders, 74, a veteran senator from Vermont, is not expected to do well in South Carolina and commentators say he has struggled to gain traction.