Internal and external economic policy advisers are trying to help Sen. Elizabeth Warren (D-Mass.) design a way to finance a single-payer Medicare-for-all health-care system that would place every American on a government insurance program.

Warren has promised more details within weeks, but her team faces a challenge in crafting a plan that would bring in large amounts of revenue while not scaring off voters with big middle-class tax increases.

The proposal could cost more than $30 trillion over 10 years. Complicating matters, she has already committed all of the money she would raise from a new wealth tax, close to $3 trillion over 10 years, to several other ideas, including child care and student debt cancellation. This has limited the Warren campaign’s options for finding additional sources of revenue without affecting middle-class Americans.

Bharat Ramamurti, a longtime Warren aide who served on the Senate Banking Committee, is widely seen as spearheading the single-payer financing plan for the campaign, according to two people who spoke on the condition of anonymity to discuss internal planning.

“They want to figure out — with one go — how to stop the ‘How are you going to pay for it?’ question,” one outside economic adviser said, speaking on the condition of anonymity to freely discuss the campaign’s thinking. “She wants something airtight but easy to understand.”

A big question facing her next decision is whether the plan can be designed in a way that finances new health benefits without imposing large new costs on middle-class families.

Other presidential candidates from both parties have often floated economic proposals, both spending increases and tax cuts, but refused to fill in key details. President Trump, for example, promised in 2016 to eliminate the federal debt but promised to wall off Medicare, Medicaid and Social Security from cuts, a platform that many budget experts said was mathematically impossible.

Since becoming president, Trump has stuck to his promise to protect Medicare from cuts but has sought to cut Medicaid and Social Security benefits. And the debt has grown markedly in the past two years, rising above $22 trillion.

While other candidates have sought ambiguity, Warren has prided herself on the granularity of her ideas. This has brought her Medicare-for-all plan under sharper scrutiny, raising the stakes surrounding her current challenge.

As she has risen in the polls, her Democratic rivals have pressed her to explain how she would finance the health-care plan.

Warren, who opposed single-payer in her 2012 Senate run, said her Medicare-for-all financing plan has been “months and months” in the making. She is widely expected to release it ahead of the next Democratic presidential debate. A campaign spokeswoman would not share any details of what is under consideration but said a plan would be released in the next several weeks. The spokeswoman added the campaign has made no final decisions.

Several ideas have surfaced as Warren faces the high-stakes decision.

Robert Pollin, a left-leaning economist at the University of Massachusetts at Amherst who has worked with the Warren and Sen. Bernie Sanders (I-Vt.) teams, said he believes two-thirds of the single-payer fund can be raised by redirecting existing public health-care spending from Medicare, Medicaid and the Department of Veterans Affairs. Pollin refused to discuss any details related to his conversations with Warren’s campaign.

Pollin suggests that the remaining third be raised by a $600 billion annual “gross receipts” tax on businesses, which he says would be less than the $650 billion firms currently spend on health care; a 3.75 percent sales tax on “nonnecessities” that exempts low-income households, to raise an additional $200 billion; and a 0.38 percent tax on wealth above $1 million, which he says would raise the remaining $200 billion.

Robert C. Hockett, a Cornell University professor who has also advised Warren and Sanders, said he has urged Warren’s team to propose financing Medicare-for-all in part with a “public premium” that would function similarly to a tax. Under this idea, Warren would propose raising revenue for a Medicare-for-all fund from a premium charge that would go to the government rather than a private insurance company.

Medicare-for-all supporters say their plan would save most Americans money by using the bargaining power created by having one government insurer to force health care providers to lower prices.

“I told them what they should do is say, we will have a low premium that will replace a high premium. Or, if you prefer, a low public tax to replace a high private tax,” Hockett said. “That would neutralize the word ‘tax’ because it would remind people that a tax is a synonym for a premium, and then at that point you are quibbling over words.”

Warren’s team has also received recommendations to adopt a “progressive consumption tax” to help fund the proposal, according to a person with knowledge of the suggestion and spoke on the condition of anonymity. This plan would raise trillions of dollars for the new national health-care system by taxing consumption of goods and services, and could exempt those at the bottom of the income distribution to be less onerous on the working class than the “value-added tax” common in Europe.

A fourth idea that has been suggested is for Warren to sell her Medicare-for-all plan as a tax cut, by comparing how much families save by having their health-care costs lowered, according to another independent economic adviser to the Warren campaign, who spoke on the condition of anonymity to speak candidly.

It is unclear if any of these proposals is likely to ultimately be proposed by Warren’s campaign.

Warren has risen in the polls over the past several months but has faced attacks from her rivals over her position on Medicare-for-all. Warren told a local television anchor during her 2012 Senate campaign that she does not support single-payer, according to remarks recently reported by Politico.

But Warren co-sponsored Sanders’s Medicare-for-all bill in 2017 and stuck with it even as other Democratic presidential candidates who backed the bill have since distanced themselves from that plan. At one presidential debate, Warren said she is “with Bernie” on the issue.

But unlike Sanders, Warren has refused to say whether taxes for the middle class would go up under her plan. Single-payer is extremely difficult if not impossible to pay for by taxing the rich alone, according to both liberal and conservative economists. Sanders says middle-class families would still come out ahead under his plan because their spending on health care would vanish.

Sanders has released a suite of potential options to finance Medicare-for-all, including a 4 percent payroll tax hike that would affect the working class, but he has not committed to one financing mechanism.

The proposal would amount to the largest transformation of the U.S. health-care system in the country’s history, although its chances of passing are unlikely even if Democrats took complete control of the federal government.

Sanders’s plan would move everyone in the country onto one government insurer, including the approximately 30 million people without insurance and the more than 150 million people who receive their health care from their employer. His bill would also provide them free medical, dental and vision care with no premiums, deductibles or co-payments.

Critics have raised objections about Sanders’s proposal, including the potential political ramifications of mandating that millions of people with employer-based care move to a government system.

How to finance the new universal system has become the latest lighting rod in the debate. Warren’s more centrist rivals have attacked her for not acknowledging single-payer would lead to middle-class tax hikes, with Pete Buttigieg, the mayor of South Bend, Ind., calling her position on the question “extremely evasive.”

But the public appetite for dramatic action on health care may be strong. A survey released this week found that health care is the most important financial for Americans in the 2020 election. And that may mean voters are willing to accept the trade-offs of a Medicare-for-all model, should Warren be able to convince them it will ultimately help their bottom line.

“It drives me nuts when people say this is so complicated. It’s not,” Pollin said. “The basics are very simple.”