Vermont Senator Bernie Sanders assailed Hillary Clinton as financially beholden to Wall Street and unable to be tough on the industry as they faced off Sunday in the most adversarial Democratic presidential debate yet.

Sanders, who's built his campaign on populist anger directed at the big banks, charged that he and the former secretary of state have many differences on Wall Street and “the first difference is I don’t take money from big banks, I don’t get personal speaking fees from Goldman Sachs.”

Clinton was paid by $675,000 for three speeches she gave to the firm in 2013 and her husband, former President Bill Clinton, has also been paid for appearances at Goldman events. She's also raised millions of dollars in campaign contributions from employees of banks, hedge funds, and other financial services companies.

She ignored Sanders' comments on the speaking fees, touting herself as building on the accomplishments of the Obama administration, while Sanders argued for a broader shift from the status quo.

Sanders is calling for a broader overhaul of laws and wants to enact an updated version of the Glass-Steagall Act, repealed while Bill Clinton was in the White House, while Clinton supports more incremental change building on the Dodd-Frank Act, signed into law by President Barack Obama.

Clinton argued that the gap between her and Sanders is not particularly large. “There’s not daylight on the basic premise that there should be no bank too big to fail and no individual too powerful to jail,” she said at the NBC-YouTube debate in Charleston, South Carolina, the final face-off before the Iowa caucuses on Feb. 1.

Then, referencing an ad released by the Sanders campaign this week that her team described as an attack on her and Obama, she said that “where we disagree is the comments that Senator Sanders has made that don’t just affect me, I can take that, but he’s criticized President Obama for taking donations from Wall Street and President Obama has led our country out of the great recession.”

Embracing Obama

In the state where Obama she and the president had an acrimonious battle during their 2008 battle for the Democratic nomination, Clinton defended her former rival and boss. “Senator Sanders called him weak, disappointing. He even in 2011 publicly sought someone to run in a primary against President Obama.”

The third candidate in the debate, former Maryland Governor Martin O'Malley, interjected that he would be stronger. “The truth of the matter, Secretary Clinton, you do not go as far in reining in Wall Street as I would,” he said. Clinton responded by noting that O'Malley fundraised on Wall Street while leading the Democratic Governors Association.

Clinton and Sanders clashed over gun control and the future of health insurance in the final Democratic presidential debate before the Iowa caucuses, with Clinton warning that Sanders’ hours-old plan to move to universal Medicare coverage risks further dividing the nation and jeopardizing the gains under the Affordable Care Act.

Clinton and Sanders Spar Over ‘Medicare for All’

“What a Medicare-for-all program does is finally provide in this country health care for every man woman and child as a right,” Sanders said, noting that even after the expanded coverage created by Obama's Affordable Care Act, “29 million people still have no health insurance.”

Clinton countered that Sanders' plan is impractical. Noting that it took the Democratic Party six decades to enact the Affordable Care Act, which she called “one of the greatest accomplishments” of the Obama administrations, she said: “To tear it up and start over again pushing our country back  into that kind of contentious debate I think is the wrong direction.”

Gun Control

In a debate that took place across the street from the Emanuel African Methodist Episcopal Church, where nine people were shot and killed by a white supremacist last year, Clinton also pressed Sanders with a laundry list of pro-gun votes he has taken in Congress.

Sanders, who said he has a D-minus rating from the National Rifle Association, protested the characterization of his record. 

“I think Secretary Clinton knows that what she says is very disingenuous,” he said. “This should not be a political issue. We should be working together.”

Clinton responded with a lengthy recounting of Sanders's record, arguing that “he has voted with the NRA, with the gun lobby numerous times.”

She continued, as Sanders looked down at his lectern: “He voted against the Brady Bill five times. He voted for what we call the Charleston loophole. He voted for immunity for gun makers and sellers, which the NRA said was the most important gun legislation in 20 years. He voted to let guns go on to Amtrak, guns go into national parks. He voted against doing research to figure out how we can save lives.”

Sanders has been noncommittal on whether he would support a bill to close the so-called Charleston loophole. “We are going to take a look at that as well,” he said Sunday on ABC's This Week.

Clinton has been questioning Sanders' vote in favor of a 2005 bill protecting gun manufacturers and sellers from legal liability for months, and on Saturday night, the Sanders campaign said he would support a proposed bill that would do so, but would propose an amendment aimed at protecting mom-and-pop gun shops. During pre-debate interviews, Clinton called it a “flip-flop.”

On stage, she was gentler in her analysis. “I am pleased to hear that Senator Sanders has reversed his position on immunity and I look forward to him joining with those members of Congress who have already introduced legislation,” she said. “There is no other industry in America that was given the total pass that the gun makers and dealers were and that needs to be reversed.”

O'Malley countered that neither Clinton nor Sanders had been sufficiently firm on guns. “I have to agree with both of them. They’ve both been inconsistent on this issue.”

Guns and health care were two of the biggest flash points between Clinton and Sanders. 

In a bold move aimed at overtaking Clinton, Sanders released a sweeping plan for single-payer health care coverage, which he is dubbing “Medicare for All,” about two hours before the start of the debate. The face-off already had promised to be a high-stakes one for both leading contenders: While national polls show Clinton continuing to lead Sanders, recent statewide surveys suggest the Vermont senator has the potential to deal embarrassing defeats to Clinton in the first two heats of the nominating contest, in Iowa and New Hampshire.

“We have the momentum; we're on a path towards victory,” Sanders said on the debate stage.

Health Care

On health care, the two candidates' differences succinctly summarizes the contrast in their approaches: Sanders, a self-described socialist, is embracing a government-run plan that Congress has biennially rejected and that even his home state of Vermont failed to enact. Clinton, who learned the power of the insurance lobby as first lady when she tried and failed to enact a reform of the health coverage system, takes what she describes as a more pragmatic approach that builds on the existing private health care system.

Though it will require new taxes on employers and wealthy Americans, Sanders said his plan will save middle-class families money immediate and will save the country money in the long run. The program would cost $1.38 trillion annually.

While Clinton's lead has nearly disappeared in the Hawkeye State—she led 42 percent to 40 percent in the Bloomberg Politics/Des Moines Register Iowa Poll released this week—and Sanders leads in New Hampshire polling, Clinton still holds a decisive edge in national polling. In a NBC News/Wall Street Journal survey released Sunday, Clinton had the support of 59 percent of Democratic primary voters, Sanders had the support of 34 percent of those surveyed, and O'Malley had the backing of 2 percent.

In South Carolina, where a majority of the Democratic primary electorate is African-American, Clinton had a 36 percentage point lead over Sanders in the most recent poll of likely voters, conducted in December by CBS News.

—Arit John contributed to this report.