Sanders, Warren and Julián Castro were the three finalists for the group’s nod, and none of those candidates won the necessary 60 percent in the first vote count. On a second ballot between Sanders and Warren, the Vermont senator secured 75 percent of the tally.

Brian Fallon, a former national press secretary for Hillary Clinton’s 2016 campaign, called the endorsement a “coup.”

Aimee Allison, founder of the organization She the People, said, “this group of women of color has spoken.”

The Center for Popular Democracy Action’s leaders sit on the national advisory board of the Working Families Party, which endorsed Warren in September. Epps-Addison said on social media that the Working Families Party is her “forever political home” and there is “no discord here, we remain completely aligned in the goal of defeating neoliberalism.”

Sanders has enjoyed something of a comeback in the months following his heart attack, winning endorsements from Rep. Alexandria Ocasio-Cortez, National Nurses United and the Los Angeles teachers union.

While courting the group, Sanders met with the leaders of the organization and its affiliates several times, according to an aide. He is planning to talk again with an affiliate in Nevada on Tuesday.

Rep. Rashida Tlaib, a member of the so-called Squad who endorsed Sanders, also spoke with the Center for Popular Democracy Action over the phone on his behalf.

Faiz Shakir, Sanders' campaign manager, said his call for a "political revolution" and his own history as an activist connected with the group. He also said the fact that the organization strongly supports “Medicare for All” was helpful.

"We spent a lot of time, both in smaller settings and larger settings, and I think a lot of people in CPD would attest that they saw a side of Bernie that they haven't seen,” he said. "In my mind, it's a side that's always there, but often times he gets characterized as a certain kind of personality that doesn't show the three dimensions of his character."