Sen. Bernie Sanders Bernie SandersNYT editorial board remembers Ginsburg: She 'will forever have two legacies' Two GOP governors urge Republicans to hold off on Supreme Court nominee Sanders knocks McConnell: He's going against Ginsburg's 'dying wishes' MORE (I-Vt.) is facing mounting scrutiny from fellow White House hopefuls for refusing to detail how he would pay for his signature “Medicare for All” plan.

Sanders’s Democratic rivals are ramping up their attacks on him as he surges to the top of polls in early voting states like Iowa and New Hampshire. The criticisms are focused on the Vermont senator’s lack of explanation over the funding mechanism for a health care proposal that is estimated to cost $32 trillion over 10 years.

“As one of the leaders of the effort for Medicare for All said, ‘I don’t know how much it’s going to cost, I don’t know what it’s going to be, we’re just going to have to do it,’ ” former Vice President Joe Biden Joe BidenBiden leads Trump by 36 points nationally among Latinos: poll GOP set to release controversial Biden report Can Donald Trump maintain new momentum until this November? MORE told an Iowa crowd on Monday, referring to Sanders but without naming him. “Well, that’s not a likely way to pass something through the Senate, saying, ‘I don’t know how much it’s going to cost.’ ”

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Former South Bend, Ind., Mayor Pete Buttigieg Pete ButtigiegBipartisan praise pours in after Ginsburg's death Bogeymen of the far left deserve a place in any Biden administration Overnight Defense: Woodward book causes new firestorm | Book says Trump lashed out at generals, told Woodward about secret weapons system | US withdrawing thousands of troops from Iraq MORE, a top contender with Biden among the more moderate candidates, joined in the criticism on Monday.

“Call me simplistic, but I think when you put forward a plan, you ought to put forward how to pay for it, too,” Buttigieg told reporters when asked about Sanders. “And it is striking that there’s been no explanation of how this is supposed to work when it could be one of the biggest things done to the American economy in a generation.”

Sanders has refused to provide more details on how to pay for his plan.

When faced with sharp questions on the topic in an interview with CBS News on Friday, Sanders said “nobody knows” how much his plan would cost and that it’s “impossible to predict.”

Sanders points out that the United States already spends significantly more per person on health care than other developed countries. He argues that Medicare for All, a system of government-provided health insurance for everyone, provides opportunities for savings through more administrative efficiency and lower payments to medical providers.

“Do you know exactly what health care costs will be ... in the next 10 years if we do nothing?” Sanders said in the CBS interview. “It will be a lot more expensive than a Medicare for All single-payer system.”

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Supporters of the progressive health care proposal argue that middle-class Americans will end up saving money because the elimination of premiums and deductibles will more than cancel out the increased taxes they would have to pay.

Sanders’s plan envisions loading trillions of dollars in spending currently being undertaken by private entities onto the government’s tab, and he has not revealed a complete plan to pay for that massive switch.

His list of possible financing options, if all added together, totals about $16 trillion over 10 years, about half the projected cost of the plan.

When asked about the shortfall in an October CNBC interview, Sanders said he did not yet have to lay out details.

“You’re asking me to come up with an exact detailed plan of how every American — how much you’re going to pay more in taxes, how much I’m going to pay,” he said. “I don’t think I have to do that right now.”

Earlier in the race, the other top progressive candidate, Sen. Elizabeth Warren Elizabeth WarrenGOP set to release controversial Biden report Biden's fiscal program: What is the likely market impact? Warren, Schumer introduce plan for next president to cancel ,000 in student debt MORE (D-Mass.), was the main focus of scrutiny over paying for Medicare for All.

After months of questions, Warren released a detailed plan aiming to raise $20.5 trillion to fund Medicare for All without raising middle-class taxes.

Sanders has not faced the same level of scrutiny as Warren over how to pay for his plan. But that’s starting to change now that he’s rising in the polls, replacing Warren as the leading progressive.

Unlike Warren, Sanders has brushed off questions about how to pay for the plan. The two candidates have also diverged on how they would get the proposal through Congress, which would be no easy task.

If Republicans maintain control of the Senate, Medicare for All’s chances would die there, assuming it even makes it through a Democratic-led House.

Facing that potential scenario, Warren has laid out executive actions she said she can take to lower drug prices and other steps on health care. She has also proposed starting with a somewhat more modest public option plan before later asking Congress to approve full-scale Medicare for All.

Sanders, on the other hand, has put all of his eggs in the Medicare for All basket, declining to discuss backup plans if Republicans maintain control of the Senate or Congress otherwise declines to pass the legislation.

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Providing details can come at a cost, many strategists argue.

Brad Bannon, a Democratic strategist, said the follow-up funding plan offered by Warren might have hurt her in the polls.

“It basically just opened her up for attack,” he said, while Sanders has been able to “float above that.”

The holes in Sanders’s plan, however, make it difficult for him to answer some key questions.

Experts note that Sanders’s prediction that Medicare for All would save the country money on health care overall assumes total national health spending would go down.

That assumption depends in large part on how much doctors and hospitals would be paid under the proposal, something that is not specified in Sanders’s bill, said Larry Levitt, a health policy expert at the Kaiser Family Foundation.

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The legislation largely calls for the secretary of Health and Human Services to determine payments for doctors and hospitals.

Without a financing plan, “there’s no way to know who would pay more and who would pay less,” Levitt said.

“As detailed as Bernie Sanders’s damn Medicare for All bill is, there are some big things left out of it,” he added.

Larry Cohen, who chairs the Sanders-aligned outside group Our Revolution, said Medicare for All criticism from Democratic rivals is “creating the disunity that will help Trump win” and that they should realize how expensive and dysfunctional the current health care system is.

“For them to use a right-wing, Republican argument is a disgrace,” Cohen said.