After months of criticism from Democratic leaders and voters, Sen. Elizabeth Warren offered a new approach to her Medicare-for-all proposal Friday, adding an intermediate step that would let Americans participate in an optional government-run health plan before she attempted to pass a mandatory program.

Under Warren’s new plan, which she calls a “Medicare for All option,” all Americans would be eligible to participate in Medicare, but no one would have to. She would push for this initiative in her first 100 days in the White House, then make a separate effort to pass Medicare-for-all later in her first term.

It’s a clear shift for the senator from Massachusetts as she attempts to ward off criticism that her single-payer health plan is unrealistic substantively and toxic politically. Warren for months had signaled a full-throated embrace of Medicare-for-all, and this new approach significantly tempers that urgency.

That may ultimately better position Warren for a contest with President Trump if she is the Democratic nominee. But on Friday it prompted an immediate backlash from her centrist rivals as well as liberal activists.

Kate Bedingfield, deputy campaign manager for former vice president Joe Biden, called Warren’s maneuvering “a full program of flips and twists,” while Sen. Michael F. Bennet (D-Colo.) said she was “backtracking,” and Lis Smith, spokeswoman for South Bend, Ind., Mayor Pete Buttigieg, accused her of trying to “paper over a very serious policy problem.”

On the left, activists sounded the alarm that Warren’s two-step approach could halt momentum for Medicare-for-all, potentially torpedoing that goal.

“Doing this in stages creates a political danger and an opening for opponents to prevent further progress,” said Adam Gaffney, president of Physicians for a National Health Program. “The longer the rollout, the more political risk.”

But Warren suggested that her approach was the best of both worlds, providing immediate relief for many Americans without giving up the goal of a universal, government-run program.

“By the end of my first 100 days, we will have opened the door for tens of millions of Americans to get high-quality Medicare for All coverage at little or no cost,” Warren wrote in a Medium post. “But I won’t stop there. Throughout my term, I’ll fight for additional health system reforms.”

She said she hopes voters will be able to “see for themselves” that their experience with Medicare is better than with private insurance, building support for Medicare-for-all.

Experts and rivals portrayed the move as a retreat from one of Warren’s highest-profile policy positions on a matter that’s of top importance to many voters. Her position arguably now resembles the “public option” favored by many of her centrist competitors: a proposal that would allow Americans to choose a government-run program if they wanted, rather than the mandatory approach favored by Sen. Bernie Sanders (I-Vt.).

“It’s much less disruptive,” said Kenneth Thorpe, a health-care expert at Emory University. “It’s in the same spirit as offering a public option.”

Sanders, accepting an endorsement Friday from the largest nurses union in the country, attacked Warren’s approach as a postponement in the fight against a rapacious health-care industry. “Some people say we should delay that fight for a few more years — I don’t think so,” he said. “We are ready to take them on right now, and we’re going to take them on on Day One.”

Warren’s team is particularly sensitive to how Sanders’s voters perceive her proposal, since she hopes to pick up their support should he drop out of the race.

“Fundamentally, the question that will prove the wisdom or the failure of the whole calculation is: Is she still close enough” to Sanders?, said one person familiar with the Warren campaign’s thinking, who spoke on the condition of anonymity because they were not authorized to discuss details of the campaign.

“If she’s the nominee — or if people have to decide to rally between one of the two of them — has she stayed close enough that people think, ‘It’s not exactly Bernie, but it’s pretty darn close’?”

Medicare-for-all has become a major test for Warren, who has steadily risen in the polls after a rocky start. As her candidacy has gained momentum, she’s faced increasing scrutiny over how she’d pay for the program without raising middle-class taxes.

Leaders in her own party have also questioned whether she could withstand attacks in a general election accusing her of eliminating the private health insurance industry and taking insurance away from more than 150 million people.

Those concerns deepened this month when Warren proposed a $20.5 trillion financing plan for Medicare-for-all, mostly in the form of new taxes. Critics said it would result in a tax hike on the middle class — something she denied — while academics said her cost assumptions were far too optimistic.

Warren’s team carefully laid the groundwork for Friday’s proposal, reaching out to selected allies, which paid off in some quarters. Ady Barkan, an influential advocate for Medicare-for-all, said the transition plan is “smart politics and good policy.”

Rep. Pramila Jayapal (D-Wash.), who introduced the House version of a Medicare-for-all bill, called it a “smart approach.”

Centrist Democrats’ angst over Warren’s momentum is one reason why former New York mayor Mike Bloomberg is considering entering the presidential race and why former Massachusetts governor Deval Patrick recently launched his candidacy.

Patrick, on his first full day campaigning Thursday, contrasted Warren’s health-care approach with his.

“We kept learning as we went, and I think that’s what’s going to have to happen for any of the big solutions,” he said, recalling how Massachusetts officials expanded health care in the state. “I want us to have an ambitious agenda. I want that. That is the goal. The means for getting there can vary.”

Warren’s transition proposal is similar to the plan Buttigieg has outlined, which he calls “Medicare for all who want it.” Under that idea, Medicare would be open to the poor and would provide subsidies to middle-income families.

“This is definitely Warren inching over toward Buttigieg and away from Bernie on health care,” opined Nate Silver, editor in chief of the politics website FiveThirtyEight.

Warren has provided more details than Buttigieg, saying she would immediately offer free health care to about half the country, including all children and poor families. She would also lower the eligibility age for Medicare to 50 and let young people buy into “a true Medicare for All option.”

The person close to Warren’s campaign said she had not expected Medicare-for-all to become such a critical issue in the presidential race. In the early spring, all the major Democratic contenders aside from Biden signaled support for it, but a backlash has prompted several candidates to back away.

“It wasn’t really clear that we were going to be spending the fall drilling down on the details of her [health-care] plan,” said the person close to Warren.

“They wanted to make sure they were stapling themselves to Medicare-for-all,” the person said. “But it wasn’t clear that there was going to be much more to it then advocating for Bernie’s plan.”

Jeff Stein and David Weigel contributed to this report.