Wallace Watson, a 52-year-old who attended her rally over the weekend in Dubuque, Iowa, said he decided to support Ms. Warren months ago because he saw her as the only candidate who could energize both voters who backed Mr. Sanders in 2016 and those who supported Hillary Clinton, the party’s nominee. In recent months, however, he admitted that his faith in Ms. Warren had been tested.

“I think she has long-term viability, but it’s unfortunate that she has had press coverage in the last months of the year that has seemed to lessen her impact,” Mr. Watson said. “She needs to step up her campaign and make herself a viable candidate again. I’m hopeful she will.”

Ms. Warren’s campaign did not respond to a request for comment for this story. But in private conversations, her advisers acknowledge that this ideological squeeze is similar to one that doomed earlier candidates like former Representative Beto O’Rourke of Texas, Senator Kirsten Gillibrand of New York and Senator Kamala Harris of California. But unlike those candidates, who often made grand pivots in order to appease both sides, Ms. Warren is placing her bet on authenticity and hoping voters will reward her for being consistent.

But she is still making minor tweaks to her strategy. She has appeared on more national television programs in recent days, including a rare appearance on the Sunday political talk show circuit. Adam Green, who leads the Progressive Change Campaign Committee and has advised Ms. Warren, said to expect her to “make the case for electability” against Mr. Trump in the coming month, aimed at “scared progressives” who are “tentatively parked with others like Biden.”

The campaign has also amassed a roster of energetic and potentially influential surrogates, including Julián Castro, the former cabinet secretary and presidential candidate, Representatives Ayanna S. Pressley of Massachusetts and Katie Porter of California, and a collective of activists called “Black Womxn For,” all of whom are set to descend on Iowa in the coming weeks.

These surrogates may go on the offensive in ways Ms. Warren has resisted. At their joint rally in Brooklyn on Tuesday, it was Mr. Castro — not Ms. Warren — who made the most explicit case for why she was better suited than her primary opponents to beat Mr. Trump.