US Democratic presidential candidate Bernie Sanders has said the country needs a “political revolution” to rein in the power of big corporations, while accusing his rival, Hillary Clinton, of being in the pocket of Wall Street.

“Can you really reform Wall Street when they are spending millions and millions of dollars on campaign contributions and when they are providing speaker fees to individuals?” Sanders said, pointing to $200,000 he said Clinton received in 2013 to deliver a speech at investment giant Goldman Sachs.

“I don’t get personal speaking fees from Goldman Sachs,” he said Sunday night during the fourth and last Democratic debate in Charleston, South Carolina.

“You've received over $600,000 in speaking fees from Goldman Sachs in one year,” Sanders said, referring to Clinton.

Clinton, meanwhile, tried to frame herself as a natural heir to President Barack Obama on a host of issues and accused the Vermont senator of being disloyal to him.

She said Sanders "criticized President Obama for taking donations from Wall Street. Senator Sanders called [Obama] weak, disappointing; he even in 2011 publicly sought someone to run in a primary against President Obama."

Sanders rebuked the charge, saying that he and the president “are friends” and “worked together on many issues.”

Hillary Clinton (L) and Bernie Sanders argue during the fourth Democratic debate in Charleston.

However, Sanders acknowledged that he wants to move beyond Obama’s agenda.

The two candidates also sparred on the issue of gun control, with Clinton attempting to connect Sanders to the gun industry and lobby.

“He has voted with the NRA, with the gun lobby, numerous times. He voted for immunity for gun makers and sellers,” Clinton said. “There is no industry in America that was given the total pass; the gun-makers and gun dealers were, and that needs to be reversed.”

Sanders accused Clinton of politicizing the issue. “This should not be a political issue. What we should be doing is working together, and, by the way, as a senator from a rural state that has virtually no gun control, I believe I am in an excellent position to bring people together.”

Former Maryland Governor Martin O'Malley, who is trailing Clinton and Sanders in polls, said neither of them has been a leader on gun control. “They’ve both been inconsistent when it comes to this issue,” he said.

Martin O'Malley speaks during the debate. (AFP photo)

Hours before the debate, Sanders unveiled a healthcare plan for all American citizens through "Medicare for all."

The plan would see citizens pay "a 2.2 percent income-based premium" towards healthcare, while companies would pay an extra 6.2 percent of an employee's income, the senator said.

Clinton said she would continue to build up on the existing healthcare system known as Obamacare.

"We have accomplished so much already. I do not to want see the Republicans repeal it, and I don't to want see us start over again with a contentious debate."

Clinton leads in most national polls, but Sanders is a threat in key states.