After backing Bernie Sanders in 2016, the Working Families Party on Monday voted to endorse Sen. Elizabeth Warren’s (D-MA) presidential campaign.

The progressive political outfit’s move bolsters Warren’s left-wing credentials as she continues to gain ground on former Vice President Joe Biden in the polls. It also signals a growth in support for Warren among the Democratic Party’s progressive wing as she and Sanders challenge Biden's frontrunner status.

The endorsement process stretched for three months and included live question-and-answer sessions with the candidates in contention for the backing. According to the party, Warren earned almost 61 percent of the vote in the first round of ranked-choice voting, followed by Sanders on almost 36 percent. More than 80 percent of voters listed Warren and Sanders as their top two picks.

“Senator Warren strikes fear into the hearts of the robber barons who rigged the system, and offers hope to millions of working people who have been shut out of our democracy and economy,” said Working Families Party national director Maurice Mitchell. “Our job now is to help Senator Warren build the mass movement that will make her transformational plans a reality.”

Although Sanders did not earn this cycle’s endorsement, as he did in 2016, Mitchell said that the organization is glad to see both of the candidates in the race.

“We’re lucky to have two strong progressive candidates leading in this race,” said Mitchell. “Senator Warren and Senator Sanders have both shaped the ideological terrain on which this campaign is being waged. They have proven an effective team on debate stages and in the polls, and we hope that partnership continues. We’re proud to call both of them allies in the fight for a more just America.”

WFP had initially chosen six candidates to do Q&A sessions for possible endorsements, including: Warren, Sanders, Sen. Kamala Harris (D-CA), Sen. Cory Booker (D-NJ), former Housing and Urban Development Secretary Julián Castro and New York City Mayor Bill de Blasio. Harris ended up being excluded from contention when she was unable to attend the scheduled session.

The endorsement comes as Warren is set to speak in New York City on Monday night after unveiling a sweeping anti-corruption plan.