Since the October debate, when Ms. Warren’s rivals attacked her policy vision, arguing that she had not adequately explained her health care proposals and that “big, structural change” was too politically risky, her campaign has yet to fully recover. And quietly, a campaign that has made few drastic changes over the course of a year has overhauled its message as caucusing and voting approaches.

It started late last year, when Ms. Warren shortened her stump speech to include fewer policy details and leave time for more audience questions, and it continued in a speech on New Year’s Eve in Boston. “Imagine an America where the lived experience of women is reflected in committee rooms and corner offices and yes, even that really nice oval-shaped office at 1600 Pennsylvania Avenue,” she said.

Now Ms. Warren has adopted a new closing to her stump speech, speaking of the need to “fight back” for big changes to society and framing her pitch in historical terms.

“Fighting back is patriotic,” she said in Muscatine on Saturday. “It’s true. We fought back against the king in order to build this country. We fought back against slavery in order to preserve this country. We fought back against a Great Depression in order to rebuild this country. We fought back against fascism in order to protect this country. We are at our best when we take on big problems, and when we fight back.”

Ms. Warren’s allies, surrogates and advisers have touted her as the Democratic “unity candidate”: liberal enough for the progressive wing and reasonable enough for the moderate wing. She got a boost over the weekend from the endorsement of The Des Moines Register, and on Monday, allied groups such as the Working Families Party and the Progressive Change Campaign Committee released a list of more than 3,000 progressive activists backing Ms. Warren. The supporters have tweeted under the hashtag “#AllOfUs4Warren.”

Many of the endorsers made a point to name-check Mr. Sanders, but go on to explain why they were backing Ms. Warren instead.

“I voted for Bernie in 2016, and continue to admire and appreciate his fierce advocacy. But 2020 is not 2016,” one of them, Susheela Jayapal, the commissioner of Multnomah County in Oregon, said in a statement. “We need bold policy and advocacy — and we also need a president who can actually govern. A president who understands and can operate the levers of influence. A president with the skills and temperament to push, to pull, to exercise power and to build power around her.”