WASHINGTON (AP) — Elizabeth Warren’s proposal to gradually move the country to a government-funded health care system has further inflamed the debate over “Medicare for All,” likely ensuring the issue will play a significant role in this week’s Democratic presidential debate.

The Massachusetts senator announced Friday that her administration would immediately build on existing laws, including the Affordable Care Act, to expand access to health care while taking up to three years to fully implement Medicare for All. That attempt to thread the political needle has roiled her more moderate rivals, who say she’s waffling, while worrying some on the left, who see Warren’s commitment to a single-payer system wavering.

The divide could complicate plans by Democrats to turn health care into a winning issue in 2020. The party successfully took back control of the House last year by championing programs that ensure that people with preexisting medical conditions keep their insurance coverage while arguing that Republicans want to weaken such provisions. But the Medicare for All debate is more delicate as advocates including Warren grapple with concerns that a new government-run system won’t provide the same quality of coverage as private insurance — and would be prohibitively expensive.

“The Medicare for All proposal has turned out to be a real deal-breaker in who gets the Democratic nomination,” said Robert Blendon, a Harvard University School of Public Health professor whose teaching responsibilities include courses on political strategy in health policy and public opinion polling. “This is not just another issue.”

Warren’s transition plan indicates she’d use her first 100 days as president to expand existing public health insurance options. That is closer to what has been supported by former Vice President Joe Biden and Mayor Pete Buttigieg of South Bend, Indiana. Both Democratic presidential candidates have criticized Medicare for All for wiping out private insurance — something they say many Americans aren’t ready for.

Warren insists she’s simply working to expand health insurance in the short term to people who don’t have it while remaining committed to the full plan in the long run.

“My commitment to Medicare for All is all the way,” Warren said while campaigning in Iowa over the weekend.

Still, the transition signified a step toward pragmatism and an acknowledgement that the government has ways to expand health insurance coverage before embracing a universal system — something that would be difficult for any president to get through Congress. Consider that current entitlements, such as Social Security and Medicare, were phased in over years, not all at once.

“If she’s looked at it and decides the sensible thing to do in order to not cause too much disruption in employment situations and within the medical system is to gear up over three years, she's probably right,” said Cindy Wolf, a customer service and shipping manager who attended the California state Democratic Convention on Saturday in Long Beach.

Still, the move may prove politically problematic for a candidate who has long decried others settling for consultant-driven campaigns seeking incremental changes at the expense of big ideas.

Vermont Sen. Bernie Sanders is the original architect of Medicare for All and has made fighting for it the centerpiece of his 2020 White House bid. He tweeted following the release of Warren’s transition plan: “In my first week as president, we will introduce Medicare for All legislation.”

Campaigning in Nevada on Monday, California Sen. Kamala Harris said, “I believe that government should not be in a position of taking away people's choice.”

“Especially on one of the most intimate and personal decisions people can make,” Harris said, “which is about how to address their health care needs.”

The criticism from others was far sharper. Top Biden adviser Kate Bedingfield dismissed Warren’s plan as “trying to muddy the waters” by offering “a full program of flips and twists.” Buttigieg spokeswoman Lis Smith said it was a “transparently political attempt to paper over a very serious policy problem.”

It’s easy to see the issue spilling into Wednesday’s debate because Warren rode a steady summer climb in the polls to become one of the primary field’s front-runners — but no longer seems to be rising. Polls recently show her support stabilizing, though not dipping, as focus on her Medicare for All ideas intensifies.

The last two debates featured Warren failing to answer direct questions on whether she would be forced to raise middle class taxes to pay for the universal health care system she envisions. That set up a plan released two-plus weeks ago in which Warren vowed to generate $20-plus trillion in new government revenue without increasing taxes on the middle class — but that’s been decried by critics who accuse Warren of underestimating how much Medicare for All would really cost.

And, though Warren never promised to begin working toward Medicare for All on Day 1 of her administration, the release of the transition plan, which spelled out that the process will take years, has unsettled some.

Una Lee Jost, a lawyer who was holding “Bernie” signs in Chinese and English at the California Democratic Convention, called any lengthy transition to Medicare for All “a serious concern.”

“We should have implemented this decades ago,” she said.

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Associated Press writers Kathleen Ronayne and Michael R. Blood in Long Beach, Calif., and Michelle Price in North Las Vegas, Nev., contributed to this report.