(CNN) Elizabeth Warren is the presidential candidate with a plan for just about everything -- but there's one significant exception when it comes to an issue that is top of mind for so many voters: Health care .

Instead, the Massachusetts Democrat is running on a bill that she first signed on to in 2017: Vermont Sen. Bernie Sanders' "Medicare for All" single-payer legislation. "I'm with Bernie," Warren has said more than once in recent months when asked about her vision for the American health care system.

Warren's decision to wholly embrace another candidate's idea -- particularly on an issue of such outsized significance in the 2020 election -- has put her in an unfamiliar position of defending and advocating a proposal that is not uniquely hers. And she's made clear that she has no intention of authoring a comprehensive health care plan of her own during the Democratic primary contest.

Warren has gone to considerable lengths in recent months to demonstrate unwavering support for Medicare for All, in one of the clearest signs of the political alliance forged between Warren and Sanders, even as they jockey for second place behind former Vice President Joe Biden. But there are early signs of the potential risks involved in Warren's full embrace of Sanders' signature proposal -- she's opened herself up to the familiar criticism that Medicare for All is too expensive, disruptive and unrealistic.

Those dynamics were on full display at Thursday night's presidential debate, when she was asked -- again -- whether Medicare for All would result in tax hikes for the middle class. Not for the first time, Warren pivoted.

"Look, what families have to deal with is cost, total cost," Warren said on the debate stage , without directly answering the question about taxes. "And the answer is, on Medicare for All, costs are going to go up for wealthier individuals and costs are going to go up for giant corporations. But for hardworking families across this country, costs are going to go down, and that's how it should work under Medicare for All in our health care system."

Her argument is that in the aggregate, Americans will spend less on health care in a single-payer system, which would eliminate premiums paid to health insurance companies, along with deductibles and copayments. But questions about additional taxes for the middle class are not going away anytime soon, and would ramp up in intensity if Warren were to win the Democratic nomination.

A Warren aide declined to comment on questions about whether there are plans for Warren to release a health care proposal of her own -- either during the primary or in the general election campaign -- as well as about middle-class tax hikes under Medicare for All, instead referring CNN to Warren's remarks after Thursday's debate

Reassurance for progressives

If Warren is confronting criticism from colleagues to the ideological right of her, the decision to take an unequivocal stance on Medicare for All has also had the effect of reassuring some progressive admirers. In interviews with CNN, activists who vocally back Medicare for All said Warren's recent "I'm with Bernie" positioning helped assuage concerns about her earlier, more circumspect, tone on Medicare for All.

She has previously mused about "different pathways" to Medicare for All and spoken about other, less ambitious bills she had co-sponsored in the Senate. And asked by CNN's Jake Tapper at a town hall in March whether she supported eliminating private health insurance, as Medicare for All would effectively do, Warren responded that Sanders has "got a runway for that."

"I think we get everybody together. And that's what it is, we'll decide," she said. "I've also co-sponsored other bills, including expanding Medicaid as another approach that we use."

Waleed Shahid, communications director for Justice Democrats, the group best known for helping to launch Alexandria Ocasio-Cortez's successful 2018 campaign for a congressional seat from New York, said Warren's "I'm with Bernie" comments marked "an incredibly important moment for her campaign."

"I think many progressives were kind of confused and getting mixed signals from her on Medicare for All," Shahid said. "That was a big moment for her to come out, pretty full-throated, for what's in the Bernie and (Washington state Rep. Pramila) Jayapal bills, which is a single-payer system."

Asked if Warren should roll out her own version of Medicare for All, Shahid argued that such a move might be counterproductive -- both for Warren and the single-payer movement.

"I don't think she needs to put out her own plan or anything like that," he said. "In fact, putting out her own plan would be seen as competing with the existing bills that people have been rallying around and worked really hard on."

One of Warren's most ardent outside backers, Adam Green of the Progressive Change Campaign Committee, acknowledged -- on the June night she said it -- that Warren's "I'm with Bernie" declaration on the debate stage "gave anyone who prioritizes that issue a permission structure for clearly voting for her."

Michael Lighty, a Medicare for All advocate and former director of public policy for National Nurses United, the first major union to endorse Sanders in 2016, was part of the push to build support in the Senate for the 2017 Medicare for All bill.

That included a sales pitch to Warren's office, which he said initially viewed it with some skepticism.

"Her staff gave the same consideration to that policy that she does to everything, which is pretty severe scrutiny, really," said Lighty, who is supporting Sanders again in 2020. Ultimately, though, Warren came around to the legislation, and was in the room next to her colleague from Vermont when it was ultimately unveiled on Capitol Hill.

Still, Lighty said, there's a nagging question in the back of his mind.

"You have to ask, if Bernie's not there, what will she do? Because she says, 'I'm with Bernie,' and if Bernie is not there, what will she do?" he said. "So I think you have to ask those questions. I don't think I would consider her commitment and policy approach settled on Medicare for All."

Another front in the battle

The simmering skepticism among some dedicated Medicare for All activists is also rooted in a less-talked-about health care battle.

Warren, along with fellow Massachusetts Democratic Sen. Ed Markey, pushed as recently as early 2018 for a repeal of the Affordable Care Act's tax on medical devices, an industry with a hub in the Bay State. The tax, which Republicans often targeted during Obama-era legislative standoffs over the health care law, was meant to help offset some of its new costs.

A Warren aide noted to CNN that her support for repealing the tax has been contingent on lawmakers finding an appropriate source of alternative revenue.

She has since become more vocal in her criticism of big pharmaceutical and insurance companies, railing against their multibillion-dollar profits at a time when millions of Americans can't foot the bill for life-saving drugs or coverage.

And in a video released last week of her recent conversation on health care with Ady Barkan, a 35-year-old activist with ALS, Warren went a step further in assuring even the most hardline single-payer advocates. Barkan noted that Warren appeared to have become increasingly "full-throated" in her support of Medicare for All.

"My observation is that your political philosophical approach to a system failure is generally not to expand the size of the public sector, but to instead heavily regulate private sector actors and beat down their greed," Barkan said. "Was that the source of your initial hesitation on the question of getting rid of private insurance, or was it something else?"

Warren gently pushed back, saying her concerns had more to do with the "transition than on (the) end point."

But she then made a point that, for a candidate who so often argues that well-regulated markets can conquer corporate excess, amounted to a notable admission that health care was different.

"There are areas where markets just don't work, and a big part of health care is one of those," Warren said. "So the idea that we'd get a couple regulations in place, and it'll all -- it'll all sort itself out, it's just not true with health care."