Health coverage has been in the news in a big way this year, thanks to Republican-led efforts in Congress to repeal the Affordable Care Act (ACA) throughout the spring and summer. That plan ultimately failed, but both sides of the political aisle do agree on one thing: There’s a lot of room to go on improving health care access and containing medical costs. A new rallying cry has risen up among Democrats: Time for single-payer! But what does that actually mean — and what could it look like?

What does “single-payer” actually mean?

At its most basic, single-payer healthcare is the idea that one single entity — generally a government body or agency — receives and handles everyone’s healthcare bills. All treatment is paid for from a single body, which in turn is financed by taxes. Basically, there’s one insurer, everyone can use it, and it covers most or all preventative and medically necessary treatments.

That’s in contrast to the multi-payer system we currently have in the U.S., where you and one or more insurance companies split up the bills for any services you receive.

For example, if a physician bills you $1,000 for a procedure, you might be expected to pay anywhere from 10% to 50% of the cost, with your insurer picking up the remaining tab. It gets even more complicated depending what kind of treatment you’re seeking, what the specific terms of your annual deductible and out-of-pocket cap are, and how many entities are involved in providing your care, but the general principle holds.

Related: How surprise medical bills knock consumers down

Are “single-payer” and “universal coverage” the same thing?

The term “single-payer” often gets interchanged with “universal healthcare,” but no, the two are not the same. A national single-payer plan would almost certainly be universal (although the law could, for whatever reason, choose to exclude certain groups — limiting coverage to documented citizens, for example), but there are alternative ways to produce universal or near-universal coverage as well.

Germany, for example, has universal coverage through a multi-payer system that operates in a complex public/private partnership. Health care is provided by private doctors and is paid for by “sickness funds” — one of a couple hundred insurance carriers, basically — that get their funding not from taxes but from employer and employee premiums. Conceptually, it’s not unlike the system the U.S. has been heading towards since the ACA took effect in 2014.

But Germany’s plan differs from the United States in that the statutory insurance doesn’t come with enormous, ever-growing co-pays, deductibles, and co-insurance like most private American plans do. When you’re covered, you’re covered, with nominal extra fees.

That’s different from something like the UK’s National Health Service (NHS). In Britain, you can receive care in NHS hospitals with NHS doctors and nurses working directly there, as government employees. Other practitioners may own private practices, but contract with the NHS to receive payment for services rendered.

Canada has something in between: Anyone in the country can apply for public health insurance. Each province has its own insurance plan, which then reimburses providers in private practice for most services they provide.

In the U.S., we do have limited public insurance for qualified groups, through Medicare and Medicaid. One way that some single-payer advocates suggest the U.S. achieve single-payer coverage would be through expanding those programs universally; the rallying cry is “Medicare for all.”

Is “Medicare for all” single payer?

In a broad sense, yes, it’s currently the most popular proposal for implementing single-payer care in the United States. As always, though, the devil is in the details.

Medicare, New York Magazine notes, is not exactly easy to expand. It probably won’t scale very well, and it’s fairly confusing even for the millions who rely on it.

Medicare recipients also still often owe significant co-pays and other out-of-pocket costs, NY Mag adds, which pretty much defeats the goal most of those who rally around single-payer care hope to achieve. What’s more, many critical aspects of care — dental and vision care, among them — aren’t included, and many participants (seniors) supplement their Medicare with private insurance.

Realistically, then, many single-payer proposals being bandied about come closer to being a universal Medicaid expansion rather than a universal Medicare one.

However, the phrase, “Medicare for all” tends to perform better in opinion polls than alternate phrasing, the Kaiser Family Foundation has found. Democrats (the party more likely to act in favor of some kind of universal care) in particular feel negatively about terms like “socialized medicine,” where majorities feel much more warmly about “Medicare-for-all.”

Who’s in favor of single-payer?

A rapidly-growing slice of the American public, for one thing.

Data from the Pew Research Center, in June of this year, found that overall, 58% of respondents to a national survey felt that providing health care to all Americans is something the government should be responsible for. 33% of respondents overall felt that there should be a single national program, and 25% felt that it should be through a mix of public and private programs.

Among self-identified Democrats, however, the skew now leans toward universal public coverage. By June 2017, a majority — 52% — responded that they favor a single, national government program for healthcare. That’s a marked increase from just one-third of Democrats agreeing with that statement in March, 2014.

Physicians, overall, also seem to be moving in the same direction as the general public. A survey of doctors released in August found that a majority — 56% — supported the idea of moving to a single-payer healthcare system in the U.S. (In 2008, the same organization found that 58% of doctors opposed the idea.)

Doctors largely want to be in the position to do medicine, not paperwork. And the more complicated the health insurance structure is, the more time physicians have to spend on the bureaucratic things they don’t care for — not just billing, but also spending time and energy making sure the tests or medications they prescribe will actually be covered by a patient’s insurance.

“Physicians long for the relative clarity and simplicity of single-payer. In their minds, it would create less distractions, taking care of patients — not reimbursement,” an executive for the firm conducting the survey said in a statement.

Who’s opposed?

Opposition basically falls into two very broad categories: the ideological and the practical.

Many, particularly but not solely on the political right or in the Republican party, oppose single-payer care on philosophical grounds.

That same Pew survey that found 33% in favor of a national single-payer health plan also found plenty of respondents who want no part of it. Overall, 38% of those surveyed said that making sure Americans can access health care isn’t the government’s job.

Most still favor keeping Medicare and Medicaid, since those already exist, but 5% of respondents said that government should not be involved in healthcare in any way — a figure that leaps to 9-10% among self-identified Republicans.

Vox reports that Republican lawmakers are already gearing up counter-attacks to Democrats’ recent embrace of single-player bills, saying that implementing expanding Medicare or Medicaid any further would endanger the Department of Veterans Affairs and the health care veterans receive.

Meanwhile, there’s the business angle to consider. The health insurance industry is massive. In the Fortune 500, UnitedHealth Group ranks sixth, Aetna 43rd, Humana 53rd, and Cigna 70th — to say nothing of the dozens of other small, medium, and large providers in the country.

You would expect insurers to be opposed to the idea of full, federal single-payer coverage that would cut them out of the loop entirely. But insurers may be less hostile to the idea of some kind of compromise approach to universal coverage than you’d think.

Back in May, Aetna CEO Mark Bertolini said that, “we should have that debate [about single-payer] as a nation.”

He added, “instead of shouting back and forth across the stage, let’s discuss what single-payer means,” and posited (rightly) that there are many, many different ways to approach that goal, if it’s the policy goal the country wants.

How realistic is it?

Single-payer or universal coverage are certainly not in the cards as a serious policy proposal for 2017 or 2018, with the current Congress and White House we have. But the seeds of potential are being planted.

Vermont Sen. Bernie Sanders, who retains much of the popularity he gained with his unsuccessful presidential run in 2016, introduced a Medicare-for-All bill on Sept. 13.

By the time his bill was formally introduced, Sanders had picked up 16 co-sponsors among his fellow Democrats, including Sens. Tammy Baldwin (WI), Richard Blumenthal (CT), Cory Booker (NJ), Al Franken (MN), Kirsten Gillibrand (NY), Kamala Harris (CA), Martin Heinrich (NM), Mazie Hirono (HI), Patrick Leahy (VT), Ed Markey (MA), Jeff Merkley (OR), Brian Schatz (HI), Jeanne Shaheen (NH), Tom Udall (NM), Elizabeth Warren (MA) and Sheldon Whitehouse (RI).

Sanders told NPR in early August that he had no expectation that the bill would go anywhere at all. “You’re not going to see it. That’s obvious,” he told NPR. Instead, he wants to use the bill to force a discussion about the merits and potential of single-payer healthcare going forward.

Reports say Sen. Brian Schatz (HI) also plans to introduce a bill this fall that would expand Medicaid (not Medicare), allowing states to let anyone — not just low-income Americans — buy in. Basically, it would be similar to the Medicaid expansion that many states participate in under the ACA, effectively using Medicaid as a “public option” for anyone uninsured who wants it.

Schatz and Sanders each plan to co-sponsor each other’s bills, Vox reports, part of a broader plan to move the conversation forward.

It’s also becoming a tenet of some 2018 House campaigns. As Rolling Stone and the Washington Post report, challengers for seats in California, Nevada, and Wisconsin — including House Speaker Paul Ryan’s seat — have already started campaigning with an explicit platform of single-payer, universal health care.

There’s a single-player proposal currently afoot in the House, too. Rep. John Conyers (MI) introduced a bill that so far 117 of his fellow Democrats in the House support.

The Conyers bill basically doesn’t just create a public option, but expands Medicare and Medicaid into one massive public option that would basically become mandatory — and fast.

But that bill Vox notes, is skeletal at best — a sign that it stands absolutely zero chance of ever moving forward in the current Congress. It’s only 30 pages (the ACA, by comparison, famously clocked in at more than 900 pages of detail), most of which say merely that changes should be made, without specifying how.

What are the challenges?

Setting aside political and ideological obstacles to changing healthcare law at all, transitioning the U.S. from its current system to a single-payer system would come with significant logistical and practical challenges to work through.

The U.S. population is currently more than 320 million people, and changing anything for all of us at once is hard work. Our policy steers more like a stack of boulders than a sports car, with every stakeholder wanting to make their own adjustments. Major changes to federal law and regulation usually require a long, slow phasing-in process, and healthcare is one that touches literally everybody in some way.

But the biggest obstacle facing any single-payer plan is money. Lots and lots of money.

Healthcare, to be blunt, is super duper expensive. It’s not that the costs themselves would go up in a transition to a single-payer system, Vox explains — it’s that the costs are high already. About 16% of our overall economy is related to healthcare, but right now all the spending is scattered. The government pays some, states pay some, private insurers pay some, employers pay some, and end-users — all of us who seek medical care — pay some.

It’s hard to get a solid feel for just how high the spending is when it’s broken up all over. To make all of it come from a single, federal source, you’d need funding… and funding means taxes, which are yet another political land mine in the U.S., unlikely to be quickly or easily resolved any time soon.

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