Washington: Former US vice-president Joe Biden has kept alive his hopes of winning the Democratic Party's presidential nomination by scoring a resounding victory in the South Carolina primary.

The major US television networks declared Biden the winner just moments after polls closed in South Carolina on Saturday (Sunday AEDT), foreshadowing an easy win for the 77-year old.

With 99 per cent of precincts reporting, Biden was on 49 per cent of the vote compared to Vermont Senator Bernie Sanders on 20 per cent.

Speaking to supporters in Columbia, the state capital, an energised Biden said: "For all those of you who've been knocked down, counted out, left behind, this is your campaign."