At a closed-door meeting on Tuesday, Sen. Bernie Sanders (I-VT) gave a point-by-point explanation of his new single-payer health care bill to the Senate Democratic caucus.

But most of them already knew what was coming — and many had already embraced it.

Over the past several months, Sanders has convinced 24 of the biggest liberal advocacy organizations and nearly one-third of Senate Democrats to co-sponsor his updated Medicare-for-all bill. It is a sharp reversal for the party that once relegated the idea to its radical fringe: Just two years ago, Sanders couldn’t find a single co-sponsor for his bill.

“We had me. And that was it,” Sanders said in an interview in his Senate office on Tuesday.

Since then, activists and the 2016 campaign — particularly Sanders’s run in the Democratic primaries — have pushed Democrats toward a rapid embrace of single-payer. Sanders’s run revealed a large and vocal group within the Democratic Party that wants to dramatically transform Obamacare rather than merely tweak it, and several Democratic senators say that their constituents have overwhelmingly clamored for Medicare-for-all in town halls at home since President Donald Trump’s election.

But working in tandem with that amped-up external pressure is Sanders’s new inside game on Capitol Hill. Sanders’s bill emerges Wednesday as the product of a “health care working group” involving staffers from more than a dozen Senate Democratic offices, according to two sources with knowledge of it. The final product tones down the policy ambitions of Sanders’s earlier single-payer proposals in subtle but important ways that made it easier for his colleagues to come on board.

The combined result of Sanders’s carrot-and-stick approach — the implicit threat of criticism from the outside, the concessions from within — is a single-payer bill that 15 Senate Democrats have publicly said they will support, and an abrupt transformation in a political party’s health care policy position that almost nobody could have guessed even a few months ago.

Sanders, said Ben Wikler, Washington director of MoveOn.org, “showed there's a tremendous, deeply politically engaged, army of voters and donors and door knockers just waiting to be energized over this issue. And then he wooed the party by being a team player."

What Sanders’s single-payer bill actually does

Before diving into the politics of the bill, it’s worth understanding the basic framework of Sanders’s single-payer policy.

Right now, US health care is a patchwork in which Americans receive their insurance in one of five main ways. Currently, 153 million Americans — about 47 percent of the country — receive insurance from their employers. There are also government programs, most importantly Medicaid and Medicare, that combine to insure close to another 38 percent of Americans. Then there are the last two buckets: the 5 percent of Americans who receive coverage on the Obamacare exchanges and the approximately 9 percent of Americans with no insurance at all.

Sanders’s plan, like Rep. John Conyers’s (D-MI) plan in the House, would collapse all those different health care options into one government plan. That single government plan would have no cost sharing — meaning patients pay no premiums or deductibles — and involve no patient cost sharing, except for prescription drugs. (As Vox’s Sarah Kliff notes, Sanders hasn’t yet tackled the biggest question about the plan: how to pay for it.)

Under the Sanders plan, private insurance would be outlawed in almost all cases. The 153 million Americans who receive insurance from their employers would lose that coverage — and in its place would receive government insurance that would force them to pay far less out of pocket, in almost all cases, than private plans do now. The private health insurance industry would all but evaporate, and government insurance would, largely, be the only game in town.

Sanders created a single-payer bill intended to win over Senate Democrats

But while keeping that transformational objective, Sanders’s Medicare-for-all bill also scales back some typical single-payer provisions to win over more Senate Democratic supporters.

For instance, earlier single-payer bills — both Sanders’s campaign plan and Conyers’s bill — called for nearly instantaneous transformation of the health care system, moving everyone onto the government plan in about one year.

Sanders’s new plan outlines how to move everyone onto the one government plan in a way that more Senate Democrats could support. Instead of a transition of just one year, Sanders’s plan would take big but more gradual steps in its first year — eliminating all cost sharing in Medicare, lowering its eligibility age from 65 to 55, and enrolling everyone under age 18. Still, Sanders said, “That’s a pretty big deal.”

In the second year of its implementation, Sanders’s bill would again lower the Medicare eligibility age from 55 to 45. In the third year, the age would again fall from 45 to 35.

It’s not until the fourth year of Sanders’s plan that every American would get a "Universal Medicare" insurance card to use for the insurance they need. Activists are overjoyed by the basic framework of the plan, but the revamped time frame marks a real change that perhaps makes it easier to envision how single-payer gets implemented.

“Maybe you know somebody could make the argument that tomorrow — or in a given day six months from now — we can put everybody into one program. I'm not sure that that's not correct,” Sanders told me.

Of course, this is still a massively quick transformation of the health care system. But one Senate Democratic staffer noted it’s also a sign of how Sanders has quietly moderated his hardline approach, making small but meaningful concessions to create a bill that would still create Medicare-for-all without making it so radical that Senate Democrats couldn’t support it.

Even centrist Democrats like Virginia Sen. Tim Kaine have already endorsed the idea of dropping the Medicare enrollment age to 55. Using that policy as the on-ramp to a universal Medicare program — as opposed to Conyers’s current or Sanders’s initial plans for an instantaneous, seismic shift — likely helped make it easier for so many Senate Democrats to support.

“The system’s going to have to digest that idea,” Sanders told me about the need to phase in Medicare-for-all over a few years, in an uncharacteristic turn of phrase for a senator not known for advocating gradual change. “This gives the system the time to adjust.”

Sanders’s Medicare-for-all bill makes other important policy concessions

In other areas, Sanders’s bill makes less disruptive the scale envisioned by other single-payer initiatives, particularly compared to the Conyers bill — while also creating a true single-payer system that’s mostly in line with what Sanders has been proposing for decades.

For instance, the House bill calls for the new single-payer insurer to set “global budgets” for hospitals — meaning that the government would allocate one lump sum for hospitals, which would allow them to reduce the amount they spend on billing. Sanders’s bill does not include that idea, said Adam Gaffney, an instructor of medicine at Harvard Medical School.

Another difference: Conyers’s bill calls for the elimination of all for-profit hospitals, making it essentially illegal to run a for-profit provider. To the disappointment of some activists lobbying Sanders’s office, that provision did not make it into the final version of the bill.

“I think it's fair to say that our bill is a major, major step,” Sanders said when asked why his new legislation leaves out elimination of for-profit insurers. “But it is what it is.”

Lastly, some single-payer activists like Gaffney also wanted Sanders’s bill to create government control over capital expenditures in health care — new hospital wings, for instance, that often do little to help patients get better but also have huge price tags. Sanders’s bill, unlike Conyers’s, leaves that untouched.

Sanders is trying to shield Senate Democrats co-sponsoring the bill from its price tag

While Sanders didn’t scale back the ambition of his single-payer bill, he appears to be trying to shield Senate Democrats from political criticisms, at least in part, while also working to ensure that advocacy organizations on the left would have their concerns addressed.

During the 2016 presidential campaign, for instance, abortion rights groups criticized Sanders for his single-payer plan because it didn’t repeal the Hyde Amendment, which makes it illegal for federal funds to be used for abortions. But the new bill explicitly puts repealing Hyde front and center, so Senate Democrats backing it are less likely to face blowback from abortion rights groups.

Sanders’s explanation for how the bill will be paid for also seems tailor-made for cushioning Senate Democrats from almost certain blowback over embracing what Republicans plan to characterize as “socialized medicine.”

During the presidential campaign, Sanders released a seven-point plan for raising $6 trillion for his bill. Yet in this new rollout, he will releasing different options to raise money for the bill separately — independent of the bill he’s gotten 13 Senate Democrats to co-sponsor. That will make it easier for Senate Democrats to co-sponsor the legislation and win over Sanders’s supporters, but also not co-sign their names to legislation calling for billions in new personal income taxes (as Sanders’s primary campaign plan did).

“What I can say is we are going to be listing a number of revenue-raising proposals,” Sanders said of the bill’s financing.

His bill also incorporates one provision in the bill that creates patient copayments for prescription drugs — something most single-payer activists oppose. A Sanders spokesperson said that policy was included to push patients toward using low-cost prescription drugs.

Still, the bill — and Democrats’ embrace of it — has many Republicans gleefully dreaming of attack ads. It does amount to a government takeover of a massive chunk of the American economy, the health insurance industry, and it would by design require large tax increases of some kind to finance.

Sanders will hold his firepower against Senate Democratic colleagues

For many Democrats, particularly those dreaming of a 2020 presidential run, their more immediate concern might be attacks from the left. There, too, Sanders is trying to reassure colleagues.

Single-payer is still meeting resistance in some Democratic circles, with a handful of Senate Democrats expressing opposition and a handful of others publicly saying that they have not made up their minds.

But in our interview, Sanders emphasized that he will not go after Senate Democrats who won’t sign on to the bill — an indication that he wants to leave the door open for them.

Even progressive senators still have reservations. Sen. Sherrod Brown (D-OH), widely seen as one of the most progressive senators in a red state, said that he would not co-sponsor the bill.

Sens. Kaine and Dianne Feinstein (CA) both told me they would not co-sponsor the bill, and Sens. Ron Wyden (OR), Bob Casey (PA), and Chris Van Hollen (MD) said they’re open to Sanders’s bill but haven’t made up their minds yet on it.

“My understanding is that the cost of single-payer is enormous,” Feinstein told me.

Thousands of activists and Sanders supporters in outside groups like the Democratic Socialists of America, Justice Democrats, and Our Revolution are going to ramp up attacks on Democrats who back off single-payer.

But Sanders himself is making clear that he himself does not want to alienate them.

“Am I going to go around and attack people who do not support my proposal? I don't do that, and I'm not going to do that,” he told me. “People have to make their own assessments, and the electorate decides the course of action in terms of elections. But I'm not going to be criticizing people.”

Why Bernie Sanders decided to revamp his bill to get others on board

Connecticut’s Richard Blumenthal is one of the Senate Democrats who came out for Sanders’s single-payer bill. Over seven town halls and multiple “emergency” health care meetings during the Republican health care debate, Blumenthal said, “the biggest applause line was always” for single-payer.

“People literally erupted, every time,” he told me after announcing he would co-sponsor the bill. “There is powerful, absolutely extraordinary momentum behind this idea.”

Blumenthal isn’t alone in suggesting that grassroots support for single-payer proved decisive in shifting Senate Democrats. Liberal organizers argue that Sanders’s true strength was in channeling this momentum into energy that directly reached Senate Democrats. They point to Sanders’s campaign to defend Obamacare, when after the election the Vermont senator held dozens of health care rallies with several Senate Democrats in Michigan, Ohio, and New York — and thousands of people showed up.

"Bernie helped make clear the future the Democratic Party lies in the realm of bold ideas rather than carefully wooing the median voter," Wikler, of MoveOn, said.

This is also how Sanders sees the fight, and why he’s been willing to prove just flexible enough on the details to win over more colleagues. During his presidential run, his health care bill faced a bevy of criticism from the center-left wonk class for not taking the underlying policy seriously enough. He may still face some criticism without pay-fors included in the bill.

But Sanders has never seen the fight in terms of a wonkish debate over health care policy, or reimbursement rates, or comparisons with international health care systems. “This is complicated stuff. And you know what? There’s nobody who has all of the answers. Nobody has all the answers,” he told me.

Instead, he has offered a different kind of vision of health care policy. That’s why, to him, the question has always been about whether Americans can amass the political will to do what he views as clearly correct. “What this struggle is about really, honestly, is not a health care debate,” Sanders said.

“Should health care be guaranteed to all people? Most people say yes. Are we wasting an enormous amount of money in the current system? Most people would say yes. Can we take on the drug companies and the insurance companies and Wall Street and their unlimited sums of money to influence Congress? ... That’s a major political struggle that we have to engage in.”