Bernie Sanders looks more like a frontrunner than ever in the Democratic nomination for president. And the stronger he looks, the more center-left Democrats are nervous about the Vermont senator, an anxiety can be distilled down to three words: Medicare-for-all.

Sanders’s opponents believe his plan to nationalize American health insurance is a political albatross. They’re convinced it’s just too far to the left to win in a general election and Republicans will hammer Sanders for wanting to hike taxes and take away people’s current health insurance.

The data from the 2018 midterms shows House candidates who endorsed Medicare-for-all fared worse than their peers who didn’t. The polling also shows Americans aren’t totally sold on Medicare-for-all. Approval and disapproval are pretty evenly split, voters can be swayed when they hear arguments against it, and a more moderate “public option” proposal polls better. It’s easy to look at the other Democratic presidential candidates who had signed on to Sanders’s plan, from Kamala Harris (who’s already dropped out) to Elizabeth Warren (who seems to be struggling to stay afloat), and conclude it’s a political loser.

But Sanders looks like the exception. He has the strongest favorability rating of any candidate among Democratic voters. He performs well against President Donald Trump in a hypothetical general election matchup, even with Medicare-for-all so tied to his political brand.

“It is a winner for Bernie because it is part of his brand and it feels authentic coming from him,” says Ashley Kirzinger, who helps run the polling by the Kaiser Family Foundation. “I mean, he is the reason why we are discussing it and it has been front and center during the Democratic campaign.”

Sanders was the most trusted Democratic candidate on health care even when he was polling behind former Vice President Joe Biden in the national surveys. Medicare-for-all is most popular among young voters, who are critical to Sanders’s base.

“When you say, ‘I’m for that,’ it says that ‘I’m for equity.’ It says, ‘I’m gonna fight back against the corporate establishment,” Harvard pollster Robert Blendon told me. “It’s symbolic of these other things which appeal to young liberal people.”

In a lot of ways, Medicare-for-all does seem politically dicey. But not for Bernie Sanders.

The evidence on whether Medicare-for-all is a political liability

The best empirical evidence on Medicare-for-all’s electoral chances comes from Alan Abramowitz, a political science professor at Emory University. He ran through the 2018 House election results to analyze how candidates who endorsed the Sanders plan fared compared to more moderate candidates who did not.

Looking at competitive House elections, 45 percent of Democratic candidates who supported Medicare-for-all prevailed in their race, a much lower success rate than the 72 percent of Democrats who won their race without backing single-payer health care.

Abramowitz’s conclusion, written in a column for Sabato’s Crystal Ball, was unequivocal (emphasis mine):

Democratic candidates who endorsed Medicare for All did significantly worse than those who did not. The estimated coefficient of -4.6 indicates that support for Medicare for All cost Democratic candidates in these competitive districts almost five points of vote margin — a substantial effect in a close election.

The 2020 primary taught a similar lesson to two high-profile Democratic presidential candidates. Harris struggled to answer the tough questions about eliminating private insurance and raising taxes when pressed about her support for Bernie’s bill. She dropped out before Iowa. The conventional wisdom says Warren fell from her peak in the 2020 field last fall amid scrutiny over her proposal on how to finance single-payer. She eventually tried to split the difference, saying she’d prioritize a short-term public option first and then later try to pass a version of the Sanders proposal. She has finished third and now a disappointing fourth in the first two states.

Polling data indicates why single-payer might be a liability. At first glance, Medicare-for-all does poll pretty well: 56 percent approval and 41 percent disapproval, according to the most recent Kaiser Family Foundation survey. It could be aided by some misconceptions, though: More than half of people, for example, think they’d be able to keep their health insurance plan under the single-payer system. (Under Sanders’s bill, they would not; most private insurance would be prohibited after the four-year transition period.)

But public opinion hinges on how you talk about the issue. Support dropped from 56 percent to 37 percent when voters were told the proposal would eliminate private insurance companies or raise taxes for most Americans. (Support correspondingly surged when voters heard the strongest talking points in favor of the proposal: universal coverage and lower health care costs.)

Kaiser Family Foundation

If you look at swing voters, Medicare-for-all struggles: A survey last year by KFF and the Cook Political Report found just 36 percent of those voters thought the policy was a good idea and 62 percent thought it was a bad idea.

All this evidence explains why establishment Democrats are so nervous about Medicare-for-all overall. They fear Sanders is running a quixotic campaign that would alienate voters they need to win over and be prone to demagogic attacks.

The KFF poll found that support for Medicare-for-all dropped to just 32 percent when voters were told it would threaten the current Medicare program. This is false, by the way — Sanders’s bill would actually improve benefits for existing Medicare enrollees — but that is still the message Trump and the GOP are going to deploy in 2020. They’ve already started making that case.

But while Medicare-for-all might be on shaky ground on its own, Sanders has managed to turn it into a very effective message in the Democratic race.

How Sanders has turned Medicare-for-all into a political winner

Sanders doesn’t raise any particular electability concerns compared to the other Democratic candidates, if you look the hypothetical head-to-head polls of a general election match-up with Trump. He fares about as well as Joe Biden and Mike Bloomberg, well-known centrists who don’t support Medicare-for-all, against the president.

So why wouldn’t Sanders pay a price as the owner of the Medicare-for-all issue? Two likely reasons:

1. His decades-long consistency on health care has built trust with voters on the issue.
2. Medicare-for-all motivates young voters who are critical to Sanders’s base.

In November, the Kaiser Family Foundation polled Democratic voters and Democratic-leaning independents on which 2020 candidate they trust most on health care. Even though Biden was leading the national polls at the time, and had criticized Sanders by saying his health care plan was too expensive, Sanders was handily the most trusted candidate on health care. Per the KFF poll, 29 percent of those voters said they trusted Sanders the most on health care, 21 percent said Biden, and 19 percent said Warren.

Kaiser Family Foundation

Sanders is seen overall as the most honest and trustworthy Democratic candidate, outpacing Biden and Warren by nearly 10 points by that metric in a January Fox News poll. The exit polls in Iowa and New Hampshire, where Sanders won the popular vote, showed primary voters broadly supported Medicare-for-all.

But some Democratic interests are still trying to turn the issue against Sanders. In Nevada, the most powerful union has made its opposition to Medicare-for-all clear, putting Sanders’s chances of notching a third early state win there at risk.

Sanders’s coalition is built substantially on young voters, though, and they like single-payer health care a lot. The most recent KFF tracking poll in January showed 65 percent of voters ages 18 to 29 say they support Medicare-for-all, and just 35 percent oppose, by far the strongest margin among any of the age cohorts. Democratic pollster Celinda Lake credited Sanders with galvanizing interest in single-payer among the younger voters.

“He had a whole audience who had not been really focused on it, particularly millennials,” she told me.

Sanders is successfully running on the issue where Warren, Harris, and some House candidates stumbled. He wrote the damn bill, as he likes to say.

He has also punted on answering the hard questions about how much taxes would need to be raised to pay for the program, though he acknowledges tax increases are necessary. He says he doesn’t think he needs to spell out all the details while he’s still a candidate.

Some Democrats see a double standard in how Sanders gets treated on Medicare-for-all compared to the scrutiny endured by Warren or Harris. They chafe at comments like those made by Rep. Alexandria Ocasio-Cortez, the most high-profile politician to back Sanders, who acknowledged a public option might be the most likely outcome when it comes time to pass actual legislation. The moderate candidates, and even Elizabeth Warren, have been battered by Sanders supporters for their lack of commitment to full-tilt single-payer health care.

But his perceived credibility has allowed Sanders — and Sanders alone, it seems — to sidestep the pitfalls and turn the issue into a political winner so far in the primary.

The big unknown: What happens when it’s Trump vs. Sanders

Sanders’s odds of winning the Democratic presidential nomination are about as high as they’ve ever been, per the FiveThirtyEight forecast. Looking ahead to a general election matchup, Trump seems to think Sanders’s brand of socialism, as manifested in Medicare-for-all, is a target heading into 2020.

“One hundred thirty-two lawmakers in this room have endorsed legislation to impose a socialist takeover of our health care system, wiping out the private health insurance plans of 180 million very happy Americans,” the president said in his State of the Union speech, a reference to the House version of Sanders’s proposal. “To those watching at home tonight, I want you to know: We will never let socialism destroy American health care.”

Mark Wilson/Getty Images

Trump will take his message to another audience too: older people, telling them Medicare-for-all would destroy the Medicare they depend on right now.

Even though it isn’t true, that won’t stop Trump from deploying it. Opinion research that shows seniors don’t support expanding government health insurance, as Vox’s Matt Yglesias covered:

In an important 2015 paper, Vivenkian Ashok, Ilyana Kuziemko, and Ebonya Washington investigate the question of why public support for economic redistribution has not risen since 1970 despite the large increase in economic inequality.

They show that the overall flat levels of support for redistribution actually mask significant shifts among different subgroups. In particular, African Americans and the elderly have become substantially less supportive of redistribution, while non-elderly whites have become moderately more supportive. … For senior citizens, the biggest issue is that the elderly “have grown increasingly opposed to government provision of health insurance.”

The authors posit that “older Americans worry that redistribution will come at their expense, in particular via cuts to Medicare.”

But the polling also indicates that voters generally trust Democrats over Trump on health care, even though all of the Democrats are pushing for more government insurance. And whatever margins Sanders loses with swing voters or older voters could conceivably be made up for by an increased turnout among young voters who support Medicare-for-all. (That’s why the relatively meh turnout in the 2020 primaries so far is a bigger concern for Sanders than the narrow issue of Medicare-for-all.)

“Will it be a loser overall? I am not sure,” Kirzinger said. “We know it alienates some voters, but it also motivates younger voters, and so if the Democratic candidate stops talking about it … does that mean those voters stay home?”

There are plenty of unknowns. Does the health care industry marshal its resources to stop Sanders? What do suburban voters fear most: a second term of Donald Trump or national health insurance? And does Sanders temper his message at all during the general election?

The senator has made recent comments acknowledging it will be a political challenge to achieve his agenda.

If Sanders runs on single-payer as an aspirational goal against Trump, while pounding the president for his plans to cut health care, that could mitigate red-baiting by Republicans.

And health care isn’t going to be the only issue litigated in the 2020 election. Trump will try to fearmonger over socialism, but he has his own record to defend. He’s the incumbent.

Working America, a political organizing arm of AFL-CIO, did a recent survey with working-class persuadable voters and found that while Medicare-for-all was not popular with them, the Trump tax bill was even less popular. The Obamacare repeal legislation Trump endorsed was the most unpopular major legislation in a generation.

Another KFF poll found just 12 percent of swing voters said that Trump’s opposition to Medicare-for-all would make them more likely to vote for him. Meanwhile, 9 percent said support for the plan would make them more likely to back a Democrat and 17 percent were more likely to support a Democratic candidate pushing “universal coverage.”

A look at the evidence on Medicare-for-all comes down to this: Generally, Americans actually are okay with the federal government playing a big role in providing health care, so a candidate who wants single-payer someday might not scare them as much as the elite consensus suggests. The idea is still above water in national polls. It’s not ridiculous to believe that Sanders — and probably only Sanders — could use it to his advantage in 2020.

But Democratic primary voters still have to decide if they are ready to put that question to the test.