Her centrist primary rivals say that's a few months too late.

For now, though, the Democratic presidential field has been left to wait, and speculate, on what Warren's forthcoming plan to pay for "Medicare for All" will look like.

But the arrival of the most hotly anticipated policy document in recent political memory will only mark the beginning of a new chapter in the long progressive push for a federal single-payer health care system -- and this increasingly testy nominating contest.

The debate over Medicare for All, which Sen. Bernie Sanders pushed into the mainstream during his first presidential campaign, has mushroomed into a broader test during the 2020 primary. Sen. Kamala Harris , like Warren a co-sponsor of the legislation Sanders introduced in 2017 and 2019, wavered under pressure early on in the campaign and ultimately dropped her support in favor of a less ambitious -- and expensive -- plan, which she rolled out over the summer.

Warren took a different route from her Senate colleague, doubling down on the bill and famously declaring in June at the first primary debate, "I'm with Bernie on Medicare for all." Four months on, Warren is among the frontrunners for the party's nomination and, with the target on her back growing by the day, has come under intense scrutiny for failing to give a simple answer to a complicated question: "How will you pay for it?"

Her campaign's formal response, which is expected to land some time before the next debate, is unlikely to spare Warren much criticism from her political rivals. The centrists, now headed up by her most vocal critic, South Bend, Indiana, Mayor Pete Buttigieg, are a good bet to remain focused on the increased spending required to make any major expansion work.

And the left, even if Sanders himself is unlikely to directly attack Warren, will pick through her plan for any sign of backtracking on that memorable commitment.

To date, Warren has responded to questions about middle class taxes by scrupulously avoiding the word itself, repeatedly guaranteeing only that overall "costs will go up for the wealthy and for big corporations, and for hard-working middle-class families, costs will go down," while pledging not to "not sign a bill into law that does not lower costs for middle-class families."

Warren has offered little insight into how her campaign is going about the process of drawing up its financing plan, and though outside economists have told reporters they are being consulted, their actual influence remains unknown.

Meanwhile, progressive lawmakers and policy wonks, some of whom have been turning this same Rubik's Cube for decades, are jockeying to shape the parameters of the next round of debate -- the one that will decide whether Warren's blueprint is viewed as mathematically sound and politically feasible.

There is an irony in Warren's conundrum.

Sanders, as he like to remind voters, "wrote the damn bill." But he has never offered a definitive cost estimate or locked himself into a matching "pay-for" plan, instead positing a menu of potential options -- like having employees pay a new 4% tax and employers a levy of 7.5%.

Think tanks across the ideological spectrum have filled the gap, meaning Warren not only has to decide on how to raise revenue for Medicare for All, but -- if she hasn't already -- choose from a range of cost estimates.

When asked about that on Monday, Warren stuck to her argument that Medicare for All represented the "cheapest way to make sure that everyone gets covered." When pressed on the particulars, she promised for the second time in 24 hours to deliver a plan in the coming weeks.

"The estimates on the cost of Medicare vary by trillions and trillions of dollars. And the different revenue streams for how to pay for it are many," Warren told reporters in Des Moines, Iowa.

By picking a lower end figure -- they range from around $14 to $34 trillion -- Warren could potentially argue that by shifting the new tax burden to employers, along with the richest corporations and individuals, she can make the numbers work without directly tapping the middle class. By most calculations, though, that might still not be enough to guarantee everything in the Sanders bill.

Whatever route Warren takes, the details her campaign publicizes are likely to live on well after the primary is over, no matter who wins the presidency in 2020. For now, though, her decision -- and how it is perceived by voters -- could affect her candidacy in conversations that run wider and deeper than the health care debate.

Former Vice President Joe Biden, right up there with or slightly ahead of Warren in most recent polling, has sought to cast Warren -- with the timing of her Medicare for All announcement as evidence -- as dissembling, unprepared, or both.

"It's mystifying that for someone who has put having a plan for everything at the center of her pitch to voters, Sen. Warren has decided to release a health care plan only after enduring immense public pressure for refusing to do so," a Biden campaign spokesman said late Sunday, shortly after Warren first teased the plan.

Sen. Amy Klobuchar, who has also launched a series of sharp salvos at Warren, struck a similar tone when she suggested, on the debate stage last week, that her colleague's refusal to deliver a plainly-worded answer to the tax question is symptomatic of some core mendacity.

"At least Bernie's being honest here and saying how he's going to pay for this and that taxes are going to go up," Klobuchar said on the debate stage. "And I'm sorry, Elizabeth, but you have not said that, and I think we owe it to the American people to tell them where we're going to send the invoice."

And Buttigieg, when asked at a rally in Nevada this week if taxes would go up to pay for his public option plan, which he calls "Medicare for all who want it," began with yet another shot at Warren.

"Good question, because not everyone has been answering this question," he said, before talking about his plans to undo President Donald Trump's corporate tax cuts and pointing to the savings that would come with allowing the federal government to negotiate drug prices.

There has been, however, growing pushback in Democratic circles to the cost narrative that Warren's opponents, both in the primary and the party's policy trenches, are seeking to exploit.

Writing in USA Today earlier this week, Donald Berwick, who served as administrator of the Centers for Medicare and Medicaid Services during the Obama administration, sought to turn the tables on Medicare for All's most vocal critics.

"Some candidates have attempted to sidestep the cost debate by promising to spend less and accomplish the same goals," Berwick wrote, without naming Buttigieg, Biden or anyone else by name. "These proposals, such as relying on a public option or expanding Medicare Advantage, offering private plans within Medicare, provide too few details to allow real cost comparisons. But it is unlikely they will do as much as Medicare for All would to reduce national health care spending or reduce costs for families."

Still, the reality for Warren is that her rise in the polls, coupled with the always explosive politics surrounding health care, means that her next move -- and plan -- is just as likely to boost her further as undermine the brand she's spent so long building.