Bernie Sanders and Hillary Clinton had it out over health care last night during some of the most heated moments of the fourth and final Democratic debate ahead of the Iowa caucuses on February 1. Some of the clashes centered on Sanders's new so-called "single-payer" health care proposal, which the Vermont senator released hours before the forum kicked off on Sunday in South Carolina, and which Clinton lambasted as impractical.

The self-identified democratic socialist's proposal would replace the United States' current employer-based private health care insurance system with a model shouldered on a single payer — the government — that would provide all Americans with coverage. He argues that such a system would lower health care spending by drastically reducing administrative costs while saving an average middle class family that earns a combined $50,000 income nearly $6,000 a year.

"What this is really about is not the rational way to go forward," Sanders said at the debate. "It's whether we have the guts to stand up to the private insurance companies."

But things got tense when Clinton slammed the plan, saying it would butcher the Affordable Care Act, popularly known as Obamacare, which she called "one of the greatest accomplishments of President Obama, of the Democratic Party, and of our country."

Clinton positioned herself as Obama's natural successor throughout the debate, and accused Sanders of being inconstant and disloyal to the administration, especially in criticizing the president during a radio interview in 2011. That statement led to one of the standout moments of the debate: a single look that has been dubbed the Sanders side-eye, and which quickly spread virally on social media as a meme.

The two candidates are currently polling close in early states of Iowa and New Hampshire, with Clinton slightly ahead in Iowa and Sanders enjoying an edge in the Granite State. Clinton has consistently maintained a lead in national polls; the latest NBC News/Wall Street Journal poll gives her a 25-point lead.

In dismissing Sanders's proposal, Clinton revisited the grueling uphill battle it took to pass Obamacare and her party's inability to push a taxpayer-funded health care system through Congress, even when they controlled it. She said her own plan would build on the existing health care law by reducing costs and capping prescription drug prices while shielding middle-class families from steep tax increases. She also ripped Sanders for failing in nearly a dozen attempts to introduce a similar single-payer health care plan since joining Congress in 1991.

"There are things we can do to improve it," Clinton remarked, speaking of the health care reform law, "but to tear it up and start over again, pushing our country back into that kind of a contentious debate, I think is the wrong direction."

"No one is tearing this up. We're going to go forward," Sanders insisted. "We're not going to tear up the Affordable Care Act. I helped write it. But we are going to move on top of that to a Medicaid-for-all system."

The senator claims that his plan, which he estimates to cost $1.38 trillion over a decade, will save the US $6 trillion compared with the current system. His single-payer proposal relies heavily on a 6.2 percent payroll tax paid by employers and a 2.2 percent "health care premium" on households. A levy on rich families earning more than $250,000 a year, including a tax on capital gains and dividends, would also help subsidize the plan.

A breakdown of the model's costs calculated for the campaign by Gerald Friedman, a professor of economics at the University of Massachusetts at Amherst, makes the case that employers would pay less than current private health insurance premiums that often come to 10 percent of payroll. The calculations also suggest that families would save 12 percent of their annual income from reduced co-pays and other deductibles.

"Universal health care is an idea that has been supported in the United States by Democratic presidents going back to Franklin Roosevelt and Harry Truman," Sanders said in a statement releasing the plan. "It is time for our country to join every other major industrialized nation on earth and guarantee health care to all citizens as a right, not a privilege."

A number of countries around the world have adopted different types of single-payer systems, including Australia, Canada, and the UK. America currently spends twice as much as most wealthy countries on health care, which accounts for 18 percent of the economy. The US Medicare system uses a single public fund but allows private insurers to control the market, so is considered a mixed delivery system.

Follow Liz Fields on Twitter: @lianzifields