Democratic presidential candidates are shifting their positions on “Medicare for All” as they battle for different wings of the party.

South Bend, Ind., Mayor Pete Buttigieg Pete ButtigiegBillionaire who donated to Trump in 2016 donates to Biden The Hill's Morning Report - Sponsored by Facebook - GOP closes ranks to fill SCOTUS vacancy by November Buttigieg stands in as Pence for Harris's debate practice MORE praised Medicare for All earlier this year, but now is attacking Sen. Elizabeth Warren Elizabeth WarrenOvernight Defense: Appeals court revives House lawsuit against military funding for border wall | Dems push for limits on transferring military gear to police | Lawmakers ask for IG probe into Pentagon's use of COVID-19 funds On The Money: Half of states deplete funds for Trump's 0 unemployment expansion | EU appealing ruling in Apple tax case | House Democrats include more aid for airlines in coronavirus package Warren, Khanna request IG investigation into Pentagon's use of coronavirus funds MORE (D-Mass.) for supporting the plan as he tries to become the favorite of more moderate voters who have mostly flocked to former Vice President Joe Biden Joe BidenFormer Pence aide: White House staffers discussed Trump refusing to leave office Progressive group buys domain name of Trump's No. 1 Supreme Court pick Bloomberg rolls out M ad buy to boost Biden in Florida MORE.

Warren, meanwhile, previously expressed openness to “different pathways” toward Medicare for All, including optional “buy-ins.” She has since pivoted to a full-throated defense of a single-payer system, a move that could help her avoid being outflanked from the left by Sen. Bernie Sanders Bernie SandersSirota reacts to report of harassment, doxing by Harris supporters Republicans not immune to the malady that hobbled Democrats The Hill's Morning Report - Sponsored by Facebook - Republicans lawmakers rebuke Trump on election MORE (I-Vt.).

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Those new stances were on display at last week’s Democratic debate, when Buttigieg and Warren engaged in a high-profile clash over Medicare for All, with Warren defending the progressive proposal and Buttigieg attacking it.

The shifts also illustrate how Medicare for All has emerged as perhaps the leading point of contention among the White House hopefuls, with the progressive wing doubling down on its support and more moderate candidates increasingly going on the attack.

“[Medicare for All] has become the defining ideological issue in the Democratic race,” said Democratic strategist Brad Bannon. “Support for Medicare for All means you’re a progressive, and opposition to Medicare for All means you’re a moderate.”

Buttigieg went after Warren when he argued that his plan to allow people the option of a government-run insurance plan or to keep their private plans is “just better than Medicare for All whether you want it or not.” He added that Warren’s approach would “obliterate private plans.”

But last year, Buttigieg said he supported Medicare for All.

“I, Pete Buttigieg, politician, do henceforth and forthwith declare, most affirmatively and indubitably, unto the ages, that I do favor Medicare for All, as I do favor any measure that would help get all Americans covered,” he wrote on Twitter.

He also defended single-payer as a “compromise position” on MSNBC as recently as February.

Buttigieg’s campaign argues that he has not changed his position, saying he has always supported Medicare for All as an end goal but that his optional plan allows for a gradual “glide path” in the meantime.

Warren, for her part, counterattacked at last week’s debate by deriding Buttigieg’s optional plan as “Medicare for all who can afford it.”

She called Medicare for All the “gold standard.”

But earlier this year, Warren expressed more openness to different approaches, including proposals for an optional government-run plan that people could buy into, along the same general lines of what Buttigieg is proposing now.

“One of the options that is certainly under discussion is to let employers buy into Medicare rather than having to go into the insurance business themselves or having to negotiate out on the private market,” Warren told reporters in February. “So a lot of options on the table, a lot of good things for us to talk about. But they all aim in the same direction, and that is that health care is a basic human right.”

She expressed a similar view at a CNN town hall in March, saying there are “a lot of different pathways.”

Three months later, Warren offered a stronger defense of Sanders’s bill, declaring at the first debate: “I’m with Bernie on Medicare for All.”

Anything less than 100 percent backing would have run the risk of giving Sanders an opening to shore up support among the party’s progressive primary voters.

Sen. Kamala Harris Kamala HarrisTexas Democratic official urges Biden to visit state: 'I thought he had his own plane' The Hill's Campaign Report: Biden on Trump: 'He'll leave' l GOP laywers brush off Trump's election remarks l Obama's endorsements A game theorist's advice to President Trump on filling the Supreme Court seat MORE (D-Calif.) has also come under fire for her shifting positions on Medicare for All. In January, she famously said, “Let’s eliminate all of that,” in reference to private insurance. She later developed her own plan, which would maintain a role for private insurers.

Among the more consistent candidates are Sanders, who has championed Medicare for All for years, and Biden, a staunch opponent of Medicare for All who prefers an optional government-run plan.

Warren, under pressure over how she will pay for her main health care plan, said Sunday she would soon release a proposal detailing which taxes she would raise to finance Medicare for All.

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The cost of Medicare for All is a daunting $32 trillion over 10 years, though supporters argue that if structured the right way, the middle class would end up paying less once the elimination of premiums and deductibles is factored in.

Buttigieg and Biden argue that an optional government-run plan is much less costly and not nearly as disruptive.

“When it comes to Medicare for All, Mayor Pete is doubling down on the idea it will be a political loser in such states as Wisconsin, Michigan and Pennsylvania,” said Democratic strategist Jim Manley.

“At the start of the cycle I was hoping against hope Medicare for All wasn’t going to be a litmus test for the party,” Manley said, but noted that more and more it seems to be.

As she embraces Sanders’s position on Medicare for All, Warren has surpassed him in the polls as the top progressive candidate.

“Warren has used her support for Medicare for All and the Green New Deal to become the progressive standard bearer,” Bannon said. “Warren has already done to Sanders what Mayor Pete wants to do with Biden.”