Medicare for All, once a fringe issue, is now squarely at the center of the national discourse.

The concept of a national government-run health plan is gaining traction as many Americans, despite gains in insurance coverage, continue to struggle with costs. A growing number of progressive Democrats support Medicare for All, now a pivotal issue for the 2020 presidential campaign and a hot topic in the candidate debates.

What is Medicare for All, and how would it work? The answers depend on whom you ask. Here’s a primer on the prickly campaign issue that has huge ramifications for American families and a big chunk of the US economy.

What does Medicare for All mean?

It represents a fundamental restructuring of the US health care system, giving government a greater role in a system now dominated by private companies.

The phrase often stands for a national single-payer system, but it has become a catch-all, representing a spectrum of different ideas to expand access to health care and tackle rising costs.

Many of the Democrats running for president say they support Medicare for All — but they back different versions of it.

“It’s a broad term that has lots of meanings,’’ said Mary Ann Chirba, a professor at Boston College Law School. “It’s used as if we’re all talking about the same thing, and we’re not.’’

How would it work?

Senator Bernie Sanders of Vermont — who helped popularize the phrase Medicare for All — supports a single-payer system, with virtually no role for private insurers. The Sanders plan would create a national government-operated insurance program that would provide comprehensive coverage for all Americans and set payment rates for doctors and hospitals.

Other plans don’t go quite as far. Senator Kamala Harris of California’s proposal would allow private insurers to continue offering plans like the private Medicare Advantage coverage now available to seniors.

Then there’s the concept of the public option, which was debated a decade ago before the passage of the Affordable Care Act. It would allow those who want it to buy into a government-operated health plan — without forcing people to give up their current coverage. Former vice president Joe Biden now favors this approach.

All of these options would differ from the current US system, which is a mix of public programs — Medicare and Medicaid — and private insurers, paying a variety of different rates to medical providers.

How is Medicare for All different from Medicare?

Medicare is a federal program that covers about 60 million Americans. It’s available to Americans 65 and older, and certain people younger than 65 who have disabilities. And it’s popular — which is why supporters of a national health insurance system use the phrase Medicare for All.

Medicare today is primarily for seniors, so its benefits are designed for that age group. If Medicare were expanded to all Americans, it would need to provide benefits for children and adults under 65.

The current program has a role for private insurance companies, allowing them to sell plans that manage and coordinate Medicare benefits for seniors. Sanders-style Medicare for All would not include private insurers.

And Medicare today requires enrollees to pay premiums, copays, and deductibles. This kind of cost-sharing would go away under Sanders’s plan.

“Medicare for All is a marketing term to appeal to people who are comfortable with Medicare,’’ said John E. McDonough, professor at the Harvard T.H. Chan School of Public Health.

How much would it cost?

There is little consensus on what a national Medicare for All system would do to health care costs. Supporters say it would help to reduce costs by slashing the administrative waste that comes with a private profit-driven system. Critics say costs would only mount as more people use health care services more often.

“It’s not at all clear there would be cost savings,’’ Chirba said. “I think there would be logistical problems. I think people just are not culturally inclined to entrust so much about the financing and design of their health coverage to the federal government.’’

A 2016 report from the Urban Institute, a Washington-based think tank, estimated that a Sanders-style single-payer health care system would increase federal spending by more than $32 trillion over a decade.

Americans would pay more in taxes to fund a government-run health system. But they would pay less — or nothing at all — in premiums and other out-of-pocket costs.

What are the arguments for it, and against it?

Supporters of a national health insurance program say it would create a more just system, where all Americans have access to high-quality health care without worrying about the costs. A government-run system would be more efficient, they argue, while eliminating ills such as surprise medical bills.

Opponents of Medicare for All include political moderates, conservatives, and industry groups representing, hospitals, insurers, and pharmaceutical companies. They say it’s not feasible, or necessary, to totally restructure the massive US health care system. They argue such change would be too disruptive, thwarting health care innovation, putting hospitals in financial peril, and raising costs and wait times for care.

Fifty-one percent of the public supports a national Medicare for All system that would provide coverage to all Americans through a single government plan, according to a July poll from the Kaiser Family Foundation. That’s a slight dip from previous months. But the details matter.

“When you ask people what they know about Medicare for All, a lot of people tell us they think they’d be allowed to keep their current coverage,’’ said Karen Pollitz, senior fellow at the foundation.

“If you tell people [that] in return for Medicare for All, you have to give up your current insurance, they don’t like that.’’

How likely is the United States to implement a Medicare for All system?

The prospects appear dim. The passage of the Affordable Care Act was a bitter partisan fight, and that law called for modest change compared with the Medicare for All proposals now on the table.

Republicans in Congress have tried repeatedly to repeal the Affordable Care Act, which expanded insurance coverage by extending Medicaid to more low-income adults and creating marketplaces where individuals could buy subsidized insurance plans. Efforts are still underway to undermine and kill the law. And Democrats remain divided on Medicare for All. Even if Democrats controlled both branches of Congress and the White House, many want to uphold and improve the Affordable Care Act — not replace it with a dramatic restructuring of the health care system.

“Chances are there is no way this could pass through the United States Senate,’’ said McDonough, who advised senators when they were writing the Affordable Care Act.

But for now, the Medicare for All debate continues.

Priyanka Dayal McCluskey can be reached at priyanka.mccluskey@globe.com. Follow her on Twitter @priyanka_dayal.