Democratic presidential rivals Hillary Rodham Clinton and Barack Obama have engaged in a bitter debate.  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 that would bring change, not politics as usual, to the White House.

"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.

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.