Sen. Bernie Sanders (I-VT) is about to put Democrats’ newfound embrace of single-payer health care to the test.

With the Republican health care bill in dire straits, the Democratic Party’s most high-profile politicians have embraced Sanders’s “Medicare-for-all” rallying cry in interviews and on the stump. In the past few weeks, Sens. Kirsten Gillibrand (D-NY), Kamala Harris (D-CA), and Elizabeth Warren (D-MA) have all voiced their support for Medicare-for-all. Sen. Cory Booker (D-NJ) told Vox’s Ezra Klein in April that he believes “ultimately, in ideas like single payer or Medicare-for-all.”

Their positions mark a clear shift in the party’s center of gravity from just last year, when single-payer health care was still regarded in Democratic circles as an impossibly, impractically far-left dream. On the campaign trail in 2016, Hillary Clinton called single-payer an idea that “will never, ever come to pass.”

Now that seems out of touch with the base. Polling shows that the percentage of Democrats who want to see a single-payer system has increased by 19 points in three years.

But despite the rise of “Medicare-for-all” as a political slogan in the party, Democrats don’t have a clear plan to translate that aspiration into policy, and their efforts to implement single-payer at the state level have been rebuffed — including in blue states like Vermont and California.

Liberal Senate Democrats have thus been able to express their admiration for Medicare-for-all in abstract terms without getting behind either the failed state initiatives or a specific congressional proposal with a massive price tag.

Sanders will soon change that. The Vermont senator is expected to release his own revised Medicare-for-all bill, the path to single-payer health care. When he does so, Senate Democrats will have to make a choice they’ve thus far avoided: Are they for Medicare-for-all in practice, or just in theory?

What does Medicare-for-all mean?

It’s worth laying out all of the wildly different kinds of policy ideas could reasonably fall within the rubric of Medicare-for-all.

Interpreted in the most literal possible way, Medicare-for-all would entail extending Medicare coverage to the rest of the country. Medicare is a broadly popular program that enjoys wide support and name recognition with the public. The idea of extending Medicare to everyone is one that voters can immediately and easily understand, particularly compared with the byzantine subsidy structure of Obamacare’s private exchanges.

But that simple description obscures massive differences between potential policy paths for the idea. “Medicare-for-all is a concept with very different meanings to different people,” said Harold Pollack, a health care expert at the University of Chicago. “Within that term, you’re really talking about vastly different visions for the health care system.”

For instance, Medicare could be made available to all Americans as a buy-in, without necessarily forcing everyone off their current private plans in order to put them on the government-run system. Vox's Dylan Matthews has looked at Rep. Peter Stark's AmeriCare Health Care Act, which wouldn't immediately create a single-payer system but would automatically enroll all children and all Medicaid patients, in addition to offering a cheap public option available to employers and individuals alike that would be cheaper than private insurance and eventually drive private insurers out of the market.

A more sweeping solution would be to create a single, government-operated health insurer that would automatically enroll every American and pay for their insurance. This idea — what is typically meant by “single-payer” or “Medicare-for-all” — would make it illegal for private insurers to offer basic health insurance plans, except in highly specialized and expensive cases. In other words, these plans would force at least 150 million Americans — or everyone who gets their insurance through their employer, except for a small number who could choose to buy specialty insurance — off their current health care plans and onto the new government system.

Rep. John Conyers (D-MI) has a single-payer bill — now co-signed by about 60 percent of House Democrats — that would do roughly that. Sanders’s upcoming Senate bill is expected to adhere to a similar blueprint. These plans would face significant political obstacles, which I’ll address in the next section.

Another big bucket of policy questions is the benefit package itself. “Many people refer to single-payer as Medicare-for-all, but most of the proposals don’t actually use Medicare,” said Larry Levitt, a health policy expert at the Kaiser Foundation. “If you proposed Medicare-for-all, some people would be horrified at the level of cost sharing required.”

Medicare is a somewhat strangely designed benefit aimed at elderly beneficiaries — it would need standardization and customization to work for the rest of the country. The plans that use Medicare’s name often don’t actually use its benefits package. Unlike in real Medicare, for instance, Conyers’s bill calls for zero cost sharing, meaning the government would pay for all Americans’ premiums, copays, and deductibles, which would drive up the bill’s price tag; Sanders’s plan is likely to offer something similar.

Democrats don’t have a real Medicare-for-all plan — yet

This ambiguity over what Medicare-for-all actually means might be cleared up if Democrats had implemented a version of it at the state level that they could point to as their model. When Democrats were writing the Affordable Care Act, they could point to the private exchanges set up in Massachusetts by Gov. Mitt Romney, and draw from that example.

No similar model exists for congressional Democrats to replicate on single-payer. In California, Democratic representatives launched a push to implement single-payer at the state level — only to see their proposal shelved by the House speaker because of the state’s constitutional provision that a certain amount of funding must go to education. In Colorado, voters rejected a single-payer ballot referendum by a nearly 60-point margin. A similar push for single-payer in Vermont also ended in failure amid pushback from the business community.

There’s a reason these states struggle so much to advance single-payer: The tax hikes required are enormous, and can easily overwhelm a state’s budget. An estimate from the Urban Institute — though rejected by the Sanders team as overblown — put his campaign proposal at $32 trillion. California’s single-payer proposal came in at $400 billion per year, or double the state budget (though as Matt Bruenig pointed out, that eye-popping number misses the fact that existing government and private insurance spending in California already approaches $300 billion).

Single-payer advocates also argue that these upfront tax hikes would pay long-term fiscal dividends, in part because the government — acting as the sole insurer — could unilaterally hold down prices and thus restrain spending. “When Republicans talk about the $32 trillion cost of single payer, can every Democrat please point out the current system costs $49 trillion!” Rep. Ro Khanna (D-CA), a House progressive, recently said on Twitter.

But there’s no getting around the fact that any full single-payer proposal would entail massive short-term disruption — both in the trillions of new taxes (Vox’s Matt Yglesias looked at some potential ways to raise them) and in Americans being thrown off their current health care plans. President Obama came under heavy fire after about 1.8 million people saw their plans canceled because of the ACA; a single-payer plan would involve hundreds of millions of Americans losing their existing coverage.

This disruption is one reason Senate Democrats in particular have been quicker to get behind Medicare buy-in proposals, or a public option in which the government would compete alongside private insurers.

Single-payer advocates say part of the problem is they’ve been unable to convince heavy hitters in the Democratic Party, who say they are primarily focused on stopping the Senate Republican health bill, to help them craft a more feasible bill.

But in the absence of a workable single-payer plan to defend, Democrats have been able to talk up the idea of a single-payer system where everyone has government insurance — without having to commit to underlying policy trade-offs.

Democrats embrace Medicare-for-all. Will they endorse a $32 trillion single-payer plan?

Congressional Democrats with national aspirations are stuck in a bind. On the one hand, more than 50 percent of Democratic voters — not to mention the vast majority of Sanders supporters — believe in a single-payer health care system (even though support for it flags somewhat when the tax increases are included).

But backing a single-payer health care system is still viewed — at least in some corners of Capitol Hill — as a political risk with the wider public. For fresh evidence, skeptics point to congressional candidate Randy Bryce’s recent interview on CNN, in which he awkwardly tried defending Sanders’s plan from accusations that it would cost $32 trillion.

Republicans pounced on those comments, as it fits with their party’s greater strategy of framing their health care bill as the only alternative to socialized medicine. “The alternative is single-payer, and we’re not going down that road,” Sen. Pat Roberts (R-KS) told me last week.

Senate Majority Leader Mitch McConnell and White House press secretary Sean Spicer have also framed the Senate health bill as the only thing standing in the way of single-payer health care. And while pharmaceutical lobbies, insurers, and medical tax device companies have mostly stayed out of the current health care fight, they’d surely mobilize an all-out war against single-payer — and the Democratic Party.

So far, Democrats have tried having it both ways — expressing support for the aspiration of single-payer without committing to its difficult policy trade-offs that would open them up to attacks from Republicans and industry.

Sen. Tammy Baldwin (D-WI), for instance, recently said that she anticipates supporting Sanders’s proposal — signaling her support for the idea of Medicare-for-all — while reserving the right to change her mind based on the specifics.

.@SenatorBaldwin on "Medicare for All" proposal: "I anticipate I will" support it, but waiting til there's a bill. #WISEN pic.twitter.com/zq5tPri9HD — Jessie Opoien (@jessieopie) July 6, 2017

If Sanders proposes a version of Medicare-for-all that Senate Democrats think they can get behind — possibly one with a lower price tag, some cost sharing, and less disruption to private health insurance — then more may be willing to co-sponsor the proposal. That, in turn, could prove a big step toward unifying the left and incorporating Sanders’s movement with the Democratic Party.

On the other hand, Sanders could come up with a bill so costly and disruptive that Senate Democrats regard it as politically radioactive. And that would put them in what they’d see as a no-win situation: forced to either back a bill that Republicans can successfully demagogue or, alternatively, reject it and incur the wrath of Sanders’s millions of followers on the left. (Many still feel the sting from Sanders’s fans after voting against his proposal to import drugs from Canada.)

This is why so much is riding on the reaction to Sanders’s upcoming proposal. Will its tax hikes wind up scaring Senate Democrats off the single-payer dream? Or will it finally give them a means to back up their embrace of Medicare-for-all with a concrete policy proposal?

“I would be laughed out of the room with Democratic Party stakeholders if I said we need a bipartisan pragmatic approach to this issue. That has been totally destroyed by the Republicans’ behavior since 2010,” said Pollack, of the University of Chicago. “The question now is how far they want to try to go.”