An aggressive Mitt Romney has taken the fight to Barack Obama and has breathed new life into his struggling campaign with a solid debate performance.

President Barack Obama has won a second term in the White House, overcoming deep doubts among voters about his handling of the US economy.

Barack Obama easily won South Carolina's bitterly contested Democratic presidential primary with the help of heavy black support on Saturday, dealing a setback to rival Hillary Clinton after a week of political brawling. John Edwards finished third in a state he won during his failed 2004 race.

Democratic presidential rivals Barack Obama and Hillary Clinton tried to explain recent controversial remarks during a tense debate on Wednesday, with Obama accusing Clinton of taking political advantage of his characterisation of small-town residents.

An aggressive Mitt Romney has taken the fight to Barack Obama and has breathed new life into his struggling campaign with a solid debate performance.

President Barack Obama has won a second term in the White House, overcoming deep doubts among voters about his handling of the US economy.

Barack Obama launched aggressive attacks against Mitt Romney on jobs, energy and Libya in their second debate as he tried to reclaim momentum.

The Democratic Party has officially nominated US President Barack Obama as its candidate for the November 6 election to face Republican Mitt Romney.