Fifty-six percent of Americans want to establish Medicare for All, according to a Kaiser Family Foundation poll published in January. A Reuters survey last August found even stronger support, with 70 percent of Americans backing single-payer.

With favorability numbers like those, it's no surprise that Democrats angling for the presidency in 2020 are campaigning on single-payer. Senators Cory Booker, Kirsten Gillibrand, Kamala Harris and Elizabeth Warren all co-sponsored Medicare for All legislation introduced by Sen. Bernie Sanders last Congress, in 2017.

So is America just a presidential election away from single-payer health care — a progressive goal since the Truman administration? Not quite. Polling shows that Americans don't understand what Medicare for All entails. Only 13 percent say they've heard a lot about the plan, according to a study from NORC at the University of Chicago.

Once people learn Medicare for All would take away their private coverage, raise their taxes and result in widespread rationing, support plummets.

About half of Americans — roughly 160 million people — get health insurance through their employers. They're largely satisfied with that coverage. Surveys show that seven in ten people like their employer-sponsored health plans.

That coverage would disappear under Sanders' 2017 bill, S. 1804. The legislation makes it "unlawful for an employer to provide benefits ... that duplicate the benefits provided under this Act." It would also make duplicative individual market coverage, such as exchange plans, illegal.

Harris is on board with this idea. At an event in Iowa last month, when asked if people who like their insurance should be able to keep it, she said, "Let's eliminate all of that. Let's move on."

The Sanders plan grants the government the sole right to cover just about everything — from hospital services, primary care and prescription drugs to mental health treatment, diagnostics and oral health care.

The current Medicare and Medicaid programs would collapse into the new Sanders health plan, as would the Children's Health Insurance Program, the Defense Department's TRICARE program and the Federal Employees Health Benefits Program. The Veterans Health Administration and the Indian Health Service would remain as they are today.

The only kind of private coverage that would remain legal under Medicare for All would be for things like elective plastic surgery and laser eye surgery.

Many people don't realize that single-payer would eliminate private insurance. Fifty-five percent of Americans mistakenly think they'll be able to keep their existing coverage under Medicare for All. Once people are told it would eliminate private insurance, support falls to 37 percent.

Support also nosedives once people learn about single-payer's astronomical cost. Sanders' bill would increase federal spending by about $32 trillion over 10 years, according to studies from both the Mercatus Center and the Urban Institute. Even doubling the amount of money the federal government takes in individual and corporate income taxes would be insufficient to pay that tab.

Such tax hikes wouldn't be popular. Six in 10 Americans oppose Medicare for All once they're told it would raise taxes.

Single-payer systems inevitably lead to rationing, since governments must restrict the supply of care to control costs.

Just look at Canada's single-payer program. Last year, the median wait for a Canadian patient to receive treatment from a specialist after a referral from a general practitioner was 19.8 weeks. In rural provinces like New Brunswick, the median wait was 45.1 weeks.

Canadians even struggle to obtain routine care. Only 43 percent of Canadians were able to get a same- or next-day appointment with their family doctors the most recent time they sought medical attention, according to a 2016 survey. One in five Canadians waited more than a week.

Across the pond, the United Kingdom's 70-year old, state-run National Health Service can't keep up with demand for medical care. Four million patients with severe pain or life-threatening conditions like lung cancer are waiting for treatment. That's nearly double the figure from 2011.

Once Americans hear more about the many downsides of single-payer, they'll sour on Medicare for All. January's Kaiser Family Foundation poll found that 70 percent of respondents would oppose Medicare for All if it led to longer wait times.

Politicians who embrace single-payer are taking a considerable risk. Sure, it's relatively popular now. A majority of voters are more likely to back a candidate who supports Medicare for All, according to a recent Hill-HarrisX survey. Only a quarter said they'd be less likely to support such a candidate.

But that support could vanish fast once Americans learn the truth about Medicare for All.