Hillary Clinton stepped up attacks Monday on rival presidential hopeful Barack Obama, portraying him as a political chameleon who had shifted views on key issues such as the Iraq war and health care.

Exchanges between the two top Democratic contenders grew testy at times as Obama defended his record and charged that Clinton and her husband, former president Bill Clinton, had twisted his stands as the battle heats up for the centre-left party's presidential nomination.

Clinton's attacks were part of the former first lady's growing effort to punch holes in Obama's image as a fresh force who would bring change, not politics as usual, to the White House. He would be the first African-American president.

"It is sometimes difficult to understand what Senator Obama has said. It's just very difficult to get a straight answer, and that's what we are probing for," she said during a televised debate in South Carolina, where Democrats hold their next presidential preference vote Saturday.

"There's a set of assertions made by Senator Clinton, as well as her husband, that are not factually accurate," Obama countered. "I have been troubled by the degree to which my record is not accurately portrayed."

The sparring among the Democrats comes ahead of the vote in South Carolina, the January 29 Florida primary and the February 5 "Super Tuesday" primaries, when party loyalists in more than 20 states cast votes.

Clinton, 60, questioned the depth of Obama's opposition to the Iraq war, pointed to a newspaper interview in which he cited the late Republican president Ronald Reagan as an important politician and charged that he changed his views on how to extend health care to all Americans.

Obama, 46, gave detailed rebuttals to the attacks. He also faced fire from former US senator John Edwards, who claimed that only his and Clinton's health-care plans would provide universal coverage.

Held on the annual holiday marking the birthday of late civil rights leader Martin Luther King Jr, the debate also touched on the issue of race in US society.

Addressing a largely black audience, the three candidates on the stage - Clinton, Obama and former US senator John Edwards - agreed that further steps were needed toward racial and gender equality.

Some have called Bill Clinton the first black US president, but 2008 White House contender Barack Obama would like to see him dance first.

Obama's quip lightened up a debate in the Democratic Party about the role of race in the US presidential campaign, a prominent issue this year because the senator from Illinois is seeking to become the first African-American president.

During the candidates' debate Monday, CNN co-host Joe Johns brought up Clinton's popularity among African-Americans and author Toni Morrison's label of "our first black president."

Obama did acknowledge that Clinton, who is white, had an "enormous affinity" with US blacks.

"I would have to, you know, investigate more of Bill's dancing abilities ... before I accurately judge whether he was in fact a brother," he deadpanned, drawing a laugh from the largely black audience.

"Well, I'm sure that can be arranged," replied US Senator Hillary Clinton, the former first lady who is battling Obama to become the Democratic nominee in the November 4 presidential election.

Bill Clinton is back on the campaign trail this year supporting his wife, a role that has included sharp attacks on Obama. So far, he hasn't questioned Obama's moves on the dance floor.

Obama showed a bit of rhythm last fall with TV talk host Ellen DeGeneres, swaying lightly to a dance beat when she welcomed him to her show.

Clinton leads Obama in nationwide polls. In the state-by-state contests to choose the Democratic nominee for the November 4 presidential election, she won the January 8 New Hampshire primary and Saturday's Nevada caucuses.

Clinton spent eight years in the White House as first lady and has served seven years as a US senator for New York state. Obama has represented the Midwestern state of Illinois in the US Senate for only three years, and his record as a state senator has increasingly been scrutinized by the Clinton campaign.