Manchester, New Hampshire: US Republican presidential contenders ganged up on rival Mitt Romney and Democrat Hillary Clinton drew rebukes in combative debates ahead of the next showdown in the tight race for the White House.

In sharp exchanges on Iraq, healthcare and immigration, the top presidential contenders in each party made a desperate last bid to gain an edge days before the Tuesday primary in New Hampshire.

The state is crucial to efforts by Clinton and Romney to revitalise their campaigns after disappointing showings in Iowa.

It is the next battleground in the state-by-state fight to pick candidates for November's election to succeed President George W. Bush.

John Edwards, a former North Carolina senator, took aim at Clinton for criticising Illinois Senator Barack Obama for shifting his positions on healthcare in recent years and failing to mandate healthcare coverage for all Americans.

Edwards noted Clinton's third-place finish in Iowa, when Obama captured the first big prize of the presidential campaign and Edwards finished second on a wave of support for their call for change in Washington.

"I didn't hear these kinds of attacks from Senator Clinton when she was ahead," Edwards said.

"Every time he speaks out for change, every time I fight for change, the forces of status quo are going to attack - every single time."

Romney, fighting for his political life, was the target of repeated shots for shifting positions on Iraq war strategy and criticising his rivals in a series of political ads.

Romney finished well behind former Arkansas Governor Mike Huckabee in the Iowa contest.

When Romney, a former Massachusetts governor and wealthy venture capitalist, tried to characterise John McCain's immigration stance, the Arizona senator cut him short.

"You can spend your whole fortune on those attack ads and it still won't be true," said McCain, who is locked in a close race with Romney for the lead in New Hampshire.

McCain later told Romney they disagreed on some issues but agreed on one thing. "You are the candidate of change," he said.