But now it’s Sanders with the wind at his back. The endorsements, on display here Sunday when Rep. Rashida Tlaib and the Sunrise Movement joined him for a rally attended by more than 900 people, are giving him a jolt of momentum weeks ahead of the Iowa caucuses and supplying him with fresh volunteers in key areas.

“I’m not surprised that Bernie is increasingly consolidating support,” said Waleed Shahid, an aide for the left-wing group Justice Democrats, which has not yet endorsed a candidate. “Many of his staff have backgrounds in community organizing, his 2016 campaign helped grow the progressive movement, and he received [Rep. Alexandria Ocasio-Cortez's] endorsement, who is shaping the future of the progressive left in significant ways.”

The endorsements follow months of courting, which included the famously curmudgeonly Sanders meeting personally with organizations’ members, according to his aides and the groups.

A Sanders staffer said that they often encouraged leaders in those meetings to ask him a common refrain in movement politics: “What brought you to the work?” That prompted Sanders, who doesn’t often talk about his personal background, to open up about growing up in a paycheck to paycheck household and being a civil rights activist in his youth.

“They would hear him talk about the first time Bernie got arrested was fighting segregated housing and systemic racism in college,” said Analilia Mejia, Sanders’ national political director. “For many of these organizers, their activism began as the same way as Bernie Sanders.”

Sanders advisers said they've prioritized winning endorsements of groups that allow grass-roots members to participate, believing that he performs best in those situations. While crafting their policy proposals, campaign aides said they sought the input of groups such as People’s Action and the Center for Popular Democracy. Reps. Ilhan Omar and Tlaib also made calls to the organizations' affiliates in their districts to advocate for Sanders, according to a campaign staffer.

When the Working Families Party endorsed Warren, people familiar with the process said that her campaign had a strong political operation that had developed relationships and engaged with members. Sanders had recently shaken up his team in New Hampshire amid concerns from his supporters that he was ignoring their worries about losing the state.

After his heart attack on Oct. 1, many political insiders thought his campaign was virtually finished and that Warren would be the field’s leading progressive candidate.

Now, in addition to giving the 78-year-old candidate’s campaign a burst of youthfulness, the endorsements by groups like the Sunrise Movement are also providing him with a larger grass-roots army. People’s Action is active in Iowa, New Hampshire, Nevada and Super Tuesday states. Sunrise Movement has a field program in Iowa and New Hampshire, as well as an independent expenditure that will back Sanders. The group said it is working to get 10,000 young people in Iowa to sign cards pledging to vote.

“There were a number of candidates that did do pretty well,” said Varshini Prakash, co-founder of the Sunrise Movement, of their endorsement process. “It just ended up that Bernie by far was supported by the movement, I think, because he’s a real movement candidate.”

In some political circles, the realization that Sanders could be the progressive standard-bearer has led to increased scrutiny. Sean McElwee, co-founder of the liberal think tank Data for Progress, which has worked with Warren's and Sanders’ campaigns, said Sanders needs to explain how he is going to broaden his support in the primary in order to defeat the moderate Joe Biden.

“Given that Sanders now has a very real chance of consolidating the left, I think he needs to make the case for how he gets to 50 percent,” said McElwee, who has not endorsed a candidate. “Beating Biden means getting to 50 percent, not making your 20 percent happy.”

Sanders’ team has argued that he is working to expand the electorate by bringing out unlikely voters and non-voters, particularly those who are young and working-class. It has also said he received donations from 300,000 new contributors in the last fundraising cycle.

Another tool that the Sanders campaign used to woo progressive groups is Ocasio-Cortez’s support, underlining her political power nationally and importance to his bid. When she officially announced her support at a rally in New York City, Sanders' team invited the co-executive director of the Center for Popular Democracy, whose organization had not yet picked a favorite in the race.

“I needed to make sure that they knew they weren’t alone,” said Mejia, “that their instinct to come with Bernie was echoed with other people who are very focused on justice like they are.”