Washington (CNN) Hillary Clinton says Bernie Sanders' Medicare-for-all plan is too expensive and too reliant on the cooperation of Republican governors.

For his part, Sanders says Clinton's attacks are off-base and she's flailing as polls show him leading in Iowa and New Hampshire.

The two leading Democratic presidential candidates' escalating feud over health care has become a defining issue in the 2016 primary campaign as they approach the final debate, Saturday in South Carolina, before voting begins.

Here's a look at what the fight's all about:

What Sanders wants: Medicare for all

It's been a priority for liberals for decades -- and Sanders hasn't given up on it. The Vermont senator wants a national, single-payer health insurance system.

What Sanders has in mind is an expansion of the Medicare program to cover all citizens. Make no mistake: It's a further-reaching expansion of health insurance, and government's role in providing it, than Obamacare.

So how would Sanders pay for it? That's the rub: Sanders hasn't offered many details, and such a plan is widely estimated to cost $15 trillion.

He'd impose a new tax to fund it -- but since a Medicare-for-all program would also eliminate the need for private insurance premiums, Sanders argues that Americans would actually save money.

Sanders had pledged to reveal all the details of his tax proposals before the Iowa caucuses, but he hasn't said what the rate to pay for Medicare-for-all would be, and campaign manager Jeff Weaver backtracked on timing Monday night, saying those numbers' release will be "not necessarily before the caucuses."

Clinton campaign press secretary Brian Fallon pointed to Sanders' Medicare-for-all proposals in the Senate, where he said Sanders would have levied a 9% tax on middle-class families to pay for the program.

In an appearance on MSNBC on Wednesday, Sanders confirmed that he would levy new fees -- but didn't say what they'd be.

"We will take away and do away with private health insurance premiums and move that to public health insurance premiums," Sanders said. "Health care is not free, but we can do a lot better in lowering the cost of health care for a middle class family than is currently the case."

Clinton's beef with Sanders' idea

Their fundamental difference boils down to this: Sanders thinks a tax hike is necessary to fund a government-run Medicare-for-all program that would be cheaper than what Americans currently spend on insurance premiums. Clinton thinks those taxes would be more expensive.

"There is no way that can be paid for without raising taxes on the middle class. The arithmetic just doesn't add up," Clinton said Tuesday in Iowa. "I don't think that is the right way to go."

She has ramped up media appearances in recent days -- including three on national morning news programs Wednesday -- to hammer Sanders for so far avoiding specifics on how taxes would increase to fund his Medicare-for-all proposal.

Clinton also points out that Republican governors have obstructed more modest expansions of government-funded coverage under Obamacare. She said Tuesday that it's naive to expect those governors would cooperate with a Medicare-for-all program.

"If that's the kind of 'revolution' he is talking about, I am worried, folks," she said.

Is Clinton's criticism fair?

On process, sure: Sanders told CNN's Dana Bash twice this month that he'd release all of his tax proposals in time for the February 1 Iowa caucuses, and now that looks unlikely.

It's also accurate that 22 states rejected Obamacare's Medicaid expansion, which would have covered their low-income residents.

But Sanders' campaign is calling Clinton a hypocrite for attacking from the right on health care, pointing out that it's the exact same tactic she denounced when then-Sen. Barack Obama did it during the 2008 Democratic primary.

"Since when do Democrats attack one another on universal health care?" Clinton said in 2008 in a clip now being shared by Sanders' campaign . "I thought we were trying to realize Harry Truman's dream. I thought this campaign finally gave us an opportunity to achieve universal health care."

Sanders also said on MSNBC on Wednesday that the Clinton campaign is "factually incorrect" in asserting that Republican governors could turn down a Medicare-for-all program.

"The way the legislation that we have introduced in the past was written, and obviously is what we believe in, is that if a Republican governor doesn't want it, it will be implemented by the federal government. This is a plan for 50 states," he said.

What Clinton wants: Cost-lowering tweaks

Unlike Sanders, Clinton would keep Obamacare, but would introduce new measures to lower Americans' health care costs

"When Americans get sick, high costs shouldn't prevent them from getting better," Clinton has said. "With deductibles rising so much faster than incomes, we must act to reduce the out-of-pocket costs families face. My plan would take a number of steps to ease the burden of medical expenses and protect health care consumers."

Under Clinton's plan, insurers would cover three doctor visits a year. This would build on current Affordable Care Act rules that provide free preventative care services, such as blood pressure screenings and vaccines. The campaign estimates it would save patients more than $100 a year.

She is also pushing for a refundable tax credit of up to $5,000 for families who face out-of-pocket costs greater than 5% of their income. Currently, taxpayers can only deduct medical costs in excess of 10% of their adjusted gross income. (The threshold is 7.5% for senior citizens.)

Clinton's plan would also protect Americans from surprise medical bills by making sure they don't have to pay more than the in-network charges for care at hospitals in their plan and for emergency services. Consumers can sometimes be hit with hefty charges if they are seen by out-of-network doctors in these hospitals.

Another part of her health care agenda is reducing prescription drug prices . Among the most controversial proposals are allowing the federal government to negotiate drug prices through Medicare and letting Americans import drugs from abroad.

She would also require drug companies that receive taxpayers' support to invest in research and would eliminate corporate write-offs for direct-to-consumer advertising, instead investing the proceeds in research.

Clinton also seeks to shield Americans from big drug bills. She would require health insurance plans cap out-of-pocket prescription bills to $250 a month for patients with chronic or serious health conditions.