After weeks of buildup, Sen. Bernie Sanders has finally released his latest plan to create a single-payer health care system in the United States, tugging along 16 Democrats as co-sponsors of the Medicare-for-all legislation, many of whom appeared with him at a buoyant press conference Wednesday afternoon. On its face, the rollout was an impressive show of political support for an idea that, not so many years ago, was widely considered a patchouli-scented left-wing fantasy, on par with dragging George W. Bush before a war-crimes tribunal and cutting the defense budget in half.

But in some subtle ways, Wednesday’s health care pep rally also showed what an uphill climb Medicare for all still faces, even among Democrats.

The fact that one-third of Senate Democrats have now endorsed Sanders’ version of Medicare for all mostly affirms something that’s been obvious for a while: Thanks to America’s favorite irascible socialist, single-payer health care is now a mainstream liberal policy idea. Even more telling is the number of potential 2020 contenders who have decided to get on board with the plan. Sens. Kamala Harris, Kirsten Gillibrand, Cory Booker, and Elizabeth Warren each took turns at the podium Wednesday extolling the virtues of socialized health insurance. Such a scene that would have been utterly unimaginable eight years ago. Their support may or may not be 100 percent heartfelt, but it’s pretty clear where they think Democratic primary voters will be standing on this issue in four years.

It’s also important that these senators have planted a flag on what they mean by “Medicare for all.” For months now, Democrats have been murmuring the phrase without fully defining it. Now, they’re getting specific. The new bill would not only extend Medicare to the entire population, but—much like the plan Sanders campaigned on—make it dramatically more generous by eliminating co-pays and deductibles while adding benefits for dental and eye care. It’s a truly all-encompassing vision of publicly financed government health care. And it will be extremely hard for other Democrats to brand less ambitious ideas—even interesting, Medicare-related ones, like blowing out Medicare Advantage—as “Medicare for all.”

But the reality is that 16 Democrats did not back a fully workable single-payer plan Wednesday. At best, they backed half of one. While the Sanders bill details how a “Medicare for All” system would work, it tap dances around the all-important question of how to pay for it.

The legislation itself does not include any taxes. Instead, its authors have written up a complementary white paper titled “Options to Fund Medicare for All” with a menu of tax hikes that add up to about $16.9 trillion over a decade (which, for what it’s worth, might not actually be enough to cover the cost of a single-payer system). That might give wonks a sense of what the bill’s backers are thinking. But it definitely gives the co-sponsors a convenient out from endorsing any specific tax increase that could be used against them in a campaign ad. More importantly, at least if you’re a single-payer fan, it means they haven’t committed themselves to some of the more controversial trade-offs that would be necessary to make single-payer a reality. If four years from now Democrats win control of Washington, it’s entirely possible some of the politicians jumping on the Medicare for all bandwagon now will jump off once Congressional Budget Office scores start rolling in and they have to reckon with the actual cost, just as some Republicans have suddenly had second thoughts about repealing Obamacare now that they’ve had to write a bill.

It’s not especially surprising that Sanders & co. would choose to leave the sticky question of taxes for a later date. As the senator himself said, this legislation is just an appetizer designed to “begin the debate” about the future of health care and single-payer. The unveiling functioned as an early head count of Democrats who are at least enthusiastic about the idea in theory. At such an early stage, it would be political malpractice to alienate potential allies by forcing them to sign on to $17 trillion of carefully spelled out tax hikes when Democrats barely have enough power in Washington to rename a post office.

And, to be sure, the senators who endorsed Sanders’ bill Wednesday did take some risks. The polling on single-payer is mixed—the Kaiser Family Foundation describes support as “malleable”—and some voters are still going to hate the idea of giving up their current coverage for whatever plan Washington cooks up. Moreover, Wednesday’s bill would reimburse doctors at current Medicare rates, which would save the government money but would surely arouse opposition from hospitals and some physician groups. The fact that Medicare for all is still controversial was probably best illustrated by the fact that one of the Senate’s most reliably progressive members, Ohio’s Sherrod Brown, declined to co-sponsor it. It’s not much of a mystery why: He’s running for re-election next year in a state Donald Trump won by eight points and that has largely elected Republicans to statewide office in recent years.

But Brown’s hesitation is a sign of the challenge single-payer supporters face. If the left wants to remake the entire U.S. health insurance system from the ground up, it will need the support of purple- and red-state Democrats. And as of now, it can’t even get a died-in-the-wool, labor-loving progressive to support a fantasy bill that shunts inevitable tax hikes into a companion document. Medicare for all might be mainstream. But it’s got a long, long way to go before it becomes consensus.