WASHINGTON: Two days after her New Hampshire defeat, White House hopeful Hillary Clinton sought to regain the upper hand against her Democratic rival Bernie Sanders in Thursday's debate, denouncing the senator's proposals as unrealistic and costly.

The former secretary of state is keen to put the recent contests behind her and strike a new path as the campaign moves to Nevada and South Carolina, where Hispanics and African-Americans play key roles in the nomination battle.

Sanders, a United States (US) senator from Vermont, is looking to build on his stunning win in the Granite State by reaching out to minority groups, with whom he has struggled to build a strong support base.

On the debate stage in Milwaukee, Wisconsin, the rivals were largely in agreement about efforts to end institutional racism and improve the lives of minorities.

They instead clashed heatedly on health care and Sanders's assertion that his plan for a single-payer system would save American taxpayers money.

"Based on every analysis that I can find by people who are sympathetic to the goal, the numbers don't add up and many people will be worse off than they are right now," Clinton said.

Continuing her assault, Clinton embraced a typical Republican line of attack to demonise Sanders, saying his plans would likely increase the size of the federal government by about 40 per cent.

"We have a special obligation to make clear what we stand for, which is why I think we should not make promises we can't keep," Clinton said.

She also suggested Sanders was aiming to dismantle President Barack Obama's landmark health care program in favour of his own plan.

Sanders bristled: "I've fought my entire life to make sure health care is a right for all people," he said.

"We're not going to dismantle anything," he insisted, explaining that middle-class families would pay $500 more in taxes while receiving 10 times that amount in the reduction of health care costs.

Fighting for every vote

Clinton is aiming to get back on track in the presidential race after a razor-thin, 0.3-percentage point win in last week's Iowa caucus and suffering a harsh 22-point blowout in New Hampshire against Sanders, a self-described democratic socialist.

Clinton will be better positioned in Nevada and then South Carolina as she seeks to profit from the coalition of black and Latino voters who helped propel Obama into the White House in 2008.

But she must try to blunt Sanders's momentum without alienating the young voters, including young women, who are flocking to his "political revolution" message ─ or risk a devastating campaign implosion.

"I have no argument with anyone making up her mind about who to support," she said about her woes wooing women voters. "I just hope that by the end of this campaign, there will be a lot more supporting me."

Peeling African-Americans away from Clinton will be crucial for Sanders, especially in South Carolina where, according to exit poll data, some 55 per cent of Democratic voters in 2008 were black.

Sanders weighed in during the debate on minority issues, saying it was crucial to implement criminal justice reform and stop overpolicing of black neighbourhoods. He also sought to broaden the pro-Sanders coalition.

"We are fighting for every vote that we can get from women, from men, straight, gay, African-Americans, Latinos, Asian Americans," he said.

Thursday revealed the uphill battle ahead for Sanders, when the Congressional Black Caucus's political action committee, including a number of black lawmakers, offered a resounding and symbolically vital endorsement of Clinton.

It described her as not only the singular candidate with the experience and temperament to be president, but someone who has advocated for minority rights for decades.

"We must have a president that understands the racial divide, not someone who just acquired the knowledge recently," CBC chairman G.K. Butterfield told reporters.

Clinton, well aware of Obama's continued popularity in South Carolina among Democrats, aligned herself with the president Thursday, saying he fails to get the credit he deserves for salvaging the US economy after the financial crisis.

And she hit Sanders for calling the president weak and a disappointment, describing it as the kind of criticism "I expect from Republicans."

Sanders shook his head, and said: "Madam Secretary, that is a low blow."

With Democrats heading first to Nevada, the next Republican contest is in South Carolina, known for its bruising, bare-knuckle politics.

Bombastic frontrunner Donald Trump and rivals Ted Cruz, Marco Rubio and Jeb Bush all spent Thursday flinging verbal jibes at one another on the campaign trail.