Republican rivals target favourite Mitt Romney

Mitt Romney, the favourite for the Republican nomination was forced to defend his activities as a private equity boss and governor of Massachusetts, as the remaining five candidates debated on television.

The debate was the last chance for Romney's rivals to challenge him before South Carolina's primary on Saturday. The remaining candidates are Rick Santorum, Newt Gingrich, Rick Perry and Ron Paul.

The latest polls show Romney has a double-digit lead in South Carolina and nationally.

Romney's bid to become the Republican candidate to challenge Barack Obama in the presidential election in Novembver could become unstoppable if he wins in South Carolina. He has already bagged victories in Iowa and New Hampshire's contests.

In the past three decades, no Republican has won the party's presidential nomination without carrying South Carolina.

Earlier on Monday, former Utah Governor Jon Huntsman withdrew from the race and endorsed Romney saying the campaign had degenerated into "toxic" personal attacks.

The former Massachusetts governor was forced to defend his work at a private equity firm, Bain Capital.

Rick Perry, who is trailing in the race, called on multimillionaire Romney to release his income tax records, which he has repeatedly declined to do. The former Massachusetts governor did not respond to that challenge.

But he later suggested he might release his tax returns in April, after he was asked about the issue by one of the debate moderators, although he did not commit to do so.