When the youth-led climate action group Sunrise Movement endorsed Sen. Bernie Sanders (I-Vt.) for president on Thursday, it boosted Sanders’ efforts to cast himself as the Democrat with the strongest commitment to fighting climate change.

More than a boon to his reputation and ground game ― the relatively new group boasts an army of 10,000 mostly college-age activists ― the endorsement has solidified Sanders’ stature as the candidate of choice for progressive voters, particularly young ones.

It comes on the heels of endorsements from the Florida-based social justice group Dream Defenders; the national progressive community group network People’s Action; and the Center for Popular Democracy Action, a network of community groups that caters specifically to working-class people of color.

“My sense is that some parts of the progressive movement felt kind of unprepared by Sanders’ successes in 2016 and now feel like they don’t want to miss out on being part of it this time around,” said Waleed Shahid, a spokesman for the left-wing group Justice Democrats, which has not endorsed in the presidential primary. “There’s no question that Sanders has been a huge ally of social movements and community organizations and helped them gain a platform on the national level.”

Sanders’ hold on the progressive mantle may have seemed like it would be a given. But in September, when the Working Families Party, a network of progressive groups with a presence in over 14 states, endorsed Sen. Elizabeth Warren (D-Mass.), it looked like an activist exodus was underway.

Stephen Maturen/Getty Images Bernie Sanders speaks at a town hall in Anamosa, Iowa. A run of prominent progressive endorsements is buoying the presidential hopeful's post-heart attack resurgence.

To Sanders, the loss of that endorsement to Warren was especially biting because his political director, Analilia Mejia, was a former head of the New Jersey Working Families Party. Several people with a say in WFP’s endorsement process told HuffPost at the time that Warren’s team had simply put in more work for it, with Sanders forgoing the opportunity to address WFP delegates in a private address in which Warren and former Housing Secretary Julián Castro participated.

And the WFP endorsement crystalized a residual sense from the 2016 election that Sanders had not yet mollified critics in the activism community who considered him unduly articulate about and focused on questions of racial justice. His groan-inducing remarks at the “She the People” conference in Houston for women of color in April and his absence at the Netroots Nation conference in Philadelphia in July contributed to a sense that Sanders had missed his moment. While those activists do not directly affect votes in the early primary and caucus states, they are influential in liberal organizations and websites that help shape perceptions of candidates.

A surge in national coverage of Warren finally turned into a polling spike as the summer came to a close. By the time the mid-September debates rolled around, Sanders was enough of an afterthought that superrich individuals and members of the Democratic establishment ― elected officials, donors and operatives ― were training their fire exclusively on Warren.

Then, at the start of October, Sanders had a heart attack. For a moment, it seemed like he might be down for the count. The incident kept Sanders off the campaign trail for two weeks and raised fears among Democratic voters about his age and health.

But the prospect of losing him rallied activists to his cause.

[Alexandria Ocasio-Cortez] reminded us that he has been so consistent from his time as mayor to his time in Congress on the issues that he’s cared about. Natalia Salgado, Center for Popular Democracy Action

“It was that event that really sent a message through the organization that this is an important time and there may not be another time for him to run, and it feels like the whole campaign for him got more focused,” said Jeff Link, an Iowa-based Democratic strategist who has not endorsed in the presidential primary.

The post-heart attack endorsement of one activist, in particular, made a world of difference for Sanders: Rep. Alexandria Ocasio-Cortez (D-N.Y.), who was reportedly moved by his illness to get off the sidelines.

Ocasio-Cortez went on to headline Sanders’ comeback rally in Queens at the end of October ― the largest of any campaign during the entire primary, per campaign estimates ― and a host of packed rallies in Iowa the following month.

Her endorsement helped nudge Stacey Walker, the chair of Iowa’s Linn County board of supervisors, into Sanders’ corner. And Ocasio-Cortez also played a role in securing Sanders the endorsement of the Center for Popular Democracy Action.

“She reminded us that he has been so consistent from his time as mayor to his time in Congress on the issues that he’s cared about,” said Natalia Salgado, CPD Action’s political director. “That kind of reminder, for a lot of folks, when we’re dealing with an administration as inconsistent and unstable as the one we’re dealing with, is so incredibly refreshing but also exciting.”

And as Sanders was showing renewed vigor on the trail and the debate stage, Warren got mired in a debate over her position on “Medicare for All” that seemed to please neither centrists nor the policy’s progressive champions.

Sanders’ consistency on Medicare for All ― he would introduce it right away rather than three years into his presidency ― as well as his support for a moratorium on the deportation of undocumented immigrants were key to getting CPD Action’s blessing, according to Salgado.

Sanders’ uncompromising policy stances played a role in other endorsements. Hector Bajaras, a founder of the Deported Veterans Support House, cited Sanders’ pledge to roll back a rule requiring the deportation of any undocumented immigrant, including veterans, who commit an “aggravated felony,” in an endorsement announcement on Wednesday.

Joel Sheakoski/Barcroft Media/Getty Images Elizabeth Warren addressed throngs of supporters in Manhattan on Sept. 16, 2019. It was a high point for her campaign, coming the day the Working Families Party endorsed her.

Rachel Gilmer, a co-executive director of Dream Defenders, which emerged after the killing of Trayvon Martin in 2013 and endorsed Sanders on Wednesday, pointed to Sanders’ “transformational vision for the political system.”

When asked about his rhetoric on race during the 2016 run, Gilmer disputed critics’ arguments that it was deficient, but also conceded, “His intersectional analysis grew in 2020.”

More than from any policy though, Sanders has benefited from the warmth he exhibits in the small audiences where he has had to court his would-be endorsers, according to Salgado.

“I’m not sure exactly what happens to him when he is in a room surrounded by the people he had made a career advocating for, but something does click and there is a certain humanity that comes into play that isn’t seen on the broader stages,” Salgado said.

Warren still has her share of wins from which to take heart. In addition to the Working Families Party, she has at her side former Housing Secretary Castro, climate expert Rhiana Gunn-Wright, the group Black Womxn For, and the Progressive Change Campaign Committee, which has adopted the role of attack dog with gusto.

Other entities currently sitting out the race include the Indivisible Project, Democracy for America and MoveOn.org. The latter two groups endorsed Sanders in the last election.

In a Thursday article in The Intercept outlining progressive leaders’ efforts to preserve unity between Sanders and Warren supporters, Democracy for America revealed that it plans to launch an initiative to defeat the “corporate wing” of the Democratic Party and bolster both of the New England senators.

The Working Families Party is also keen to make clear its preference for Warren does not amount to contempt for Sanders.

“Our movements are bigger than any presidential campaign,” said Nelini Stamp, the WFP director of strategy and partnerships.

It’s not clear what kind of consequences these endorsements will have in the early states — especially in Iowa, where Sanders, Warren and Buttigieg have invested so much of their resources. People’s Action has an Iowa affiliate, the Iowa Center for Community Improvement, but most of the other groups do not.

“Maybe there are people who belong to these groups who aren’t typical voters,” Link said. “That is where it could be the most helpful.”