Throughout, Ms. Warren has kept one eye trained on policy and the other on realpolitik: protecting her aspirational brand of liberalism and robbing Republicans (and her Democratic rivals) of a potent talking point about middle-class taxes. She ignored those in the Democratic establishment who had warned her against eliminating private insurance, and ultimately settled on a proposal whose math loosely adds up but that many, even within her party, see as mostly a values statement.

Image A recent round of recent Facebook ads from the Warren campaign featured 12 bullet points of her agenda as president. It did not include Medicare for all.

The transition she announced Friday focuses her first 100 days on a public health insurance plan closer to what her more moderate rivals have championed — drawing instant condemnation that she was trying to have it both ways. “We’re not going to beat Donald Trump next year with double talk on health care,” Kate Bedingfield, a deputy campaign manager for Mr. Biden , said on Friday.

The backlash seemed to ensure that Ms. Warren will face even more pressure to explain her stance in the Democratic debate in Atlanta on Wednesday.

“Up until putting a price tag on Medicare for all she had imposed her will on the primary and dictated the tempo on her terms,” Mr. Lehane said. “This is first time that she is having to react and not follow her campaign script.”

A plan conspicuous by its absence

Ms. Warren had a plan for everything. That was her calling card almost from the start of the race. But health care was conspicuous it its absence, all the way back to her first trip to Iowa, when she barely mentioned Medicare for all. In June, Ms. Warren simply tucked into the slipstream of her liberal rival, Mr. Sanders, on his signature issue.

“I’m with Bernie on Medicare for all,” she said at the first debate.

But by fall, health care was emerging as the No. 1 issue in the campaign, and the absence of a plan was increasingly untenable. She was dogged for weeks by questions about whom she would raise taxes on, saying only that total costs for the middle class would go down.