The Medicare-for-all train is picking up steam. Fifteen Senate Democrats are co-sponsoring Bernie Sanders’s single-payer health care bill. Several of them look like presidential contenders.

Now more than ever, universal single-payer health care is being treated as a viable policy idea in the United States — in part because it is an increasingly politically popular one. But the American public is notoriously fickle when it comes to health care: Just look at how the Affordable Care Act’s approval lagged, right up until Republicans tried to repeal it.

So does the public really want single-payer health care?

“I think the honest answer is they don’t really know where they stand,” Celinda Lake, a top progressive pollster, told me this week. “There’s a lot of play in the polling, because a lot of people are still not really clear on what we’re talking about.”

That was the general consensus of the polling experts I talked to who have studied single-payer polling. Americans like the idea of universal coverage. They are ever more comfortable with a robust government role.

But they haven’t had a specific plan to digest and respond to. The trade-offs that come with any health care plan (the transition from work-based coverage to government insurance, increased taxes to pay for increased government spending) and the potent attacks that will be deployed by opponents (the government will take over your health care) have been shown to drag down support for a national health care plan.

“Lots of the country may be very ready for ... single-payer as an idea, as a slogan. They may embrace the goals that go with it: universal coverage, health care as a right,” Drew Altman, president and CEO of the Kaiser Family Foundation, one of the premier polling outfits for health care issues, told me.

“That doesn’t tell us much at all about where the country will be if you get to a debate about real legislation,” he continued, “with real details, with real winners and losers.”

The public supports an active government role in health care

First things first: Americans have long been comfortable with the idea of the government guaranteeing health insurance for every American. The idea ducked underwater when opposition to Obamacare was at its fiercest a few years ago, but it has been the majority position for most of the past two decades.

On top of that, support for a national health insurance plan provided by the government has been on a steady if slow trend upward since the 1990s. This chart from the Kaiser Family Foundation summarizes the movement:

“It’s somewhat resurgent,” Altman said. “It’s popular among Democrats, and it’s become more popular among independents.”

Indeed, independents show the greatest movement. Ten years ago 62 percent of Democrats supported single-payer, and today 64 percent of them say they do. Republicans haven’t really budged either (24 percent to 28 percent).

But among independents, support for single-payer health care has surged from 42 percent in 2008 and 2009 to 55 percent today.

The inevitable trade-offs shave off support for single-payer

It is nearly universally acknowledged that Americans might not love their current health insurance, but they are uneasy about any major disruption to the system. After all, 90 percent of Americans do have some kind of health coverage right now.

That was more or less the thesis of Paul Starr’s Remedy and Reaction, which tracks the rise and fall of various health care reform plans at the federal level over the past century. Single-payer health care, more than any plan that so far preceded it, would significantly disrupt the existing system — and whatever Americans’ complaints about their health care, that will be a real challenge for the concept.

Among the most compelling arguments against single-payer health care is that it would require increased taxes to pay for it. (It’s worth noting that Sanders’s plan is forgoing, for the time being, a detailed plan to pay for it.) Combine increased taxes with the major disruption single-payer promises for people who have insurance already, and the political challenges become evident.

“Many people in the protected public — those with good employer benefits; veterans; seniors on Medicare — will see the new taxes required for single-payer as an extra burden that doesn't benefit them,” Starr, a Princeton University professor, told me.

The counterargument used by single-payer supporters is that any increased taxes will be offset by the elimination of health insurance premiums paid by workers and their employers and, in the case of the Sanders Medicare-for-all plan, almost no more out-of-pocket costs.

Still, Starr is skeptical.

“Yes, advocates can argue to the employed that single-payer will substitute for current insurance premiums,” he said. “But those with good employer insurance would have to be confident that their employers will use all the savings from insurance to raise wages — and I'm doubtful most people will have that confidence.”

This is the debate that has not been well litigated with the public, and that’s what leads many opinion experts to say Americans aren’t really sure whether they support single-payer.

In isolation, Americans support universal coverage and even a national health care program. But once they hear the counterarguments, like higher taxes, they become less enthusiastic.

“The issue is about: Are Americans willing to pay taxes?” said Robert Blendon, professor of health policy and political analysis at Harvard University’s public health school, where he conducts polls on health care. “What’s really a problem with much of the polling is the way it’s described to people.”

The Kaiser Family Foundation has illustrated this phenomenon very clearly. In its polling, support for single-payer drops about 20 percent when people who initially said they supported the proposal are told it would give the government too much control or require Americans to pay higher taxes.

Then again, the foundation showed you can increase support by selling the ideas of health care as a basic right, lowering costs, and reducing the role of private insurance companies.

The underlying point is that we are still in the nascent stages of a truly robust public debate about single-payer health care. We don’t yet know for sure how it would shake out.

“No one should mistake where single-payer stands in today’s polling with where it would stand in a real debate,” Altman told me. “I think it’s very hard to say what the country is or isn’t ready for.”

Young voters and women are the key to single-payer health care

One thing is increasingly unambiguous, though: Democrats want single-payer health care. The Kaiser polling shows nearly two-thirds of them support it. Other polling has also found a majority of Democrats now back a national health care program for the first time.

“You’re really seeing the numbers increase in Democratic primary voters,” Lake said.

Which is why pollsters aren’t surprised to see budding presidential contenders like Sens. Kamala Harris (D-CA), Cory Booker (D-NJ), and Kirsten Gillibrand (D-NY) signing on to Sanders’s Medicare-for-all bill.

“It is absolutely a way to rally the Democratic base,” Altman said. “It’s the Democratic way to rally their base, just as ‘repeal and replace’ was the Republican way to rally their base.”

One notable trend is the surge in support among younger votes, who are not usually interested in health care as an issue. The Pew Research Center found that 45 percent of Americans ages 18 to 29 — including 66 percent of those who identify as Democrats — supported single-payer health care.

Lake explicitly credited Sanders with galvanizing interest in single-payer among younger voters.

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

Blendon said he thought that for those younger voters, single-payer health care is kind of a litmus test for a candidate’s commitment to equity and universal programs. The health care itself is, in a way, beside the point.

“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,” he said. “They are not health care voters, but essentially it’s symbolic of these other things which appeal to young liberal people.”

The other key demographic is women, who polling consistently finds are more concerned with health care than men. The Kaiser Family Foundation found last year that 41 percent of women said health care was extremely important to their vote, 10 points higher than men.

“Eighty percent of health care voters are women. Eighty percent of health care decision-makers are women,” Lake said.

And they’ll be interested in the specifics, she added. Slogans won’t suffice.

“Women are the drivers here,” she said. “They pay a lot of attention to the details. They are very, very concerned not just with what it costs but about the care that will be delivered.”

So what we find when we survey the public is a complicated picture. Americans support universal health coverage, and they are comfortable with the government playing a major role. That much we know.

But when it comes to single-payer health care specifically, people’s feelings are a little less clear. They like the goals but the trade-offs make them nervous, and it’s not yet clear whether single-payer supporters can win them over with their counterarguments to the counterarguments.