Sanders received nearly 74 percent of the ballots cast by delegates from the group’s affiliates, according to People’s Action. Goehl said there was a period in the primary when it seemed the vote “might be closer.” Sanders' rollout of his housing plan, which calls for building almost 10 million affordable housing units as well as national rent control, was a turning point, Goehl said.

The nod from People’s Action is the latest in a string of progressive endorsements nabbed by Sanders in recent months. Reps. Alexandria Ocasio-Cortez (D-N.Y.), Ilhan Omar (D-Minn.) and Rashida Tlaib (D-Mich.) backed him in October, giving a jolt of excitement to his campaign after he had been eclipsed in the national polls by Warren and suffered a heart attack.

Sanders has since leapfrogged Warren and is now in second place behind Joe Biden, according to the RealClearPolitics average. He also recently bested her in scoring an endorsement from the Center for Popular Democracy, a left-wing group that counts 600,000 members.

Analilia Mejia, Sanders’ national political director, said the backing of the two coalitions sends a message by progressives.

"Combined, it’s over 1.5 million members,” she said. “It gives you a sense of the popularity of our candidate, especially with those who are doing the work.”

The Sanders team worked for months to court People’s Action: Sanders met and spoke with the group’s affiliates, and his team convened conference calls with them while putting out policy proposals. Omar and Tlaib also contacted the organization’s leaders in their home states to vouch for the presidential candidate, according to a Sanders aide.

Omar said an event hosted by one of the group’s affiliates helped persuade her to endorse Sanders: “It was at the Iowa People's Action presidential forum in September where I saw the power of their grassroots movement. And it was clear then that Bernie was the candidate to take on Trump and take back the White House."

People’s Action has a presence in Iowa, New Hampshire, Nevada and Super Tuesday states, including in rural areas. In addition to Sanders and Warren, Pete Buttigieg, Julián Castro and Kamala Harris also responded to the group’s questionnaire.

“People’s Action is one of the great grassroots organizations in this country,” said Sanders in a video that will be released after the endorsement is announced. “I am just so proud and excited to have the endorsement of People’s Action because they understand what I understand: That at the end of the day, the only way we make real change in this country is through grassroots activism.”