Sen. Elizabeth Warren speaks at a union event in New Hampshire recently. (Brian Snyder/Reuters)

Sen. Elizabeth Warren’s campaign Friday unveiled a transition phase that would lead up to her broader Medicare-for-all plan, with the goal of establishing less far-reaching changes before the country ultimately adopts a complete government-run health system.

The proposal, which would open the current Medicare program to all Americans, is designed in part to shield Warren from attacks that she would quickly take away private insurance from more than 150 million people if elected president.

Warren intends for the transition plan — which she had signaled she would release earlier this month — to pass Congress in her first 100 days as president via a budget procedure allowing it to be approved in the Senate by a simple majority.

“I’ll give every American over the age of 50 the choice to enter an improved Medicare program, and I’ll give every person in America the choice to get coverage through a true Medicare for all option,” wrote Warren in a Medium post outlining her plan.

Warren’s campaign says she would continue to fight for Medicare-for-all as the next step. But she is risking criticism from those on the left, who could argue that once a public option is adopted there would be little political appetite to embrace the more sweeping Medicare-for-all system.

Still, Warren says that “no later than my third year in office” she would “fight to pass legislation that would complete the transition to full Medicare for all.”

For weeks, Warren has been trying to navigate between the Democratic Party’s liberal activists, who are agitating for a full-throated embrace of Medicare-for-all, and her rivals in the center of the party, who complain that Warren is promising unrealistic and chaotic change.

The transition plan is sure to ignite a new round of discussion about universal health care and whether Warren can successfully ease concerns among some Democratic voters who worry that her health-care blueprint is too sweeping to appeal to general election voters.

She also could face concerns from liberals that she is departing from the pure Medicare-for-all plan created by Sen. Bernie Sanders (I-Vt.).

The transition that Warren’s campaign announced Friday arguably has similarities to the plan that South Bend, Ind., Mayor Pete Buttigieg has outlined, which he calls “Medicare for all who want it.” Under that idea, Medicare would be opened up to the poor and would provide subsidies to middle-income families who want to enroll.

Warren’s attempts to walk this narrow path have proved difficult, threatening to mire her candidacy in the same territory that has caused problems for other candidates. Warren’s campaign was the focus of multiple attacks on health care during the last Democratic debate, leaving her trying to fend off criticism from the left and right wings of the party.

Centrist Democrats’ angst over Warren’s momentum is part of why former New York mayor Mike Bloomberg is considering entering the presidential campaign, and why former Massachusetts governor Deval Patrick recently launched his candidacy Thursday.

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

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

The issue has also been difficult for other candidates. Sen. Kamala D. Harris (D-Calif.) saw her campaign stall after she backed away from supporting Medicare-for-all and released a plan with a 10-year transition where employers could still offer private health insurance.

Sanders’s plan includes its own four-year transition, but it would clearly push Americans into the Medicare system.

One person close to Warren’s campaign said that she had not expected Medicare-for-all to become such a critical issue in the presidential race. In the early spring, all of the major contenders aside from former vice president Joe Biden had signaled support for it.

In the winter, just after launching her presidential bid, Warren said that her goal was “affordable health care for every American” and adding that there were “different ways to get there.”

But by the first presidential debate in June, Warren declared, “I’m with Bernie on Medicare-for-all.”

“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 one person close to Warren who is not authorized to discuss details of the campaign. “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.”