WASHINGTON — Elizabeth Warren has not endorsed anyone since dropping out of the 2020 race, but one of her key allies is siding with Bernie Sanders in the hopes of advancing their shared progressive agenda as rival Joe Biden consolidates moderate support.

The Working Families Party, a labor union-backed coalition of state-based groups that work to elect progressive public officials, was one of the first major left-leaning groups to pick sides in the 2020 Democratic primary last September when the group backed Warren at a massive rally in Washington Square Park in New York City.

“We said from the very beginning that there were two progressive champions in this race, and that our North Star was to elect one of them as president,” said the party's national director, Maurice Mitchell. “Now, the Working Families Party will marshal its grassroots supporters and staff to help Senator Sanders win the nomination and defeat Donald Trump.”

The organization will hold a call for Warren supporters Monday night to discuss what they should do next, featuring elected officials who had backed the Massachusetts Democrat but are now supporting Sanders, I-Vt. The group has a particularly active presence in several states that vote in upcoming primary contests, including Pennsylvania, Wisconsin, New York and Connecticut, where it hopes to help Sanders.

The Progressive Change Campaign Committee, another longtime Warren ally, urged members in Michigan to vote for Sanders in the Tuesday’s primary, but stopped short of a full endorsement.

“If your top priority is beating Trump, and you want to be super sure Bernie or Biden are tested and can go the distance if they are the nominee, a vote for Bernie is a vote to let the contest continue instead of prematurely coronating Biden,” the group said in an email to supporters Saturday.

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It's still unclear where the majority of Warren voters will end up in a two-person contest between Sanders and the former vice president.

A Morning Consult tracking poll found Biden and Sanders splitting Warren supporters roughly evenly, with 43 percent saying they’d side with the progressive candidate and 36 percent preferring the moderate.

Warren herself told MSNBC’s Rachel Maddow last week that she has no timeline on when she’ll decide on an endorsement.

Most of Warren’s more prominent endorsers, including members of Congress like Massachusetts Rep. Ayanna Pressley and surrogates like former Housing Secretary Julián Castro, are also taking their time as well.

Meanwhile, Biden earned the support in recent days of two former presidential rivals, Sens. Kamala Harris, D-Calif., and Cory Booker, D-N.J.

Time is running short. Six states will vote Tuesday and then another four big ones — Florida, Ohio, Arizona and Illinois — will vote next week, after which some analysts predict Biden’s delegate lead could become insurmountable.

Nelini Stamp, the director of strategy and partnerships at Working Families, said her group hopes its endorsement of Sanders will push others who backed Warren to consolidate around the progressive candidate

But she said the group is not trying to pressure Warren herself and acknowledged Warren's base is unlikely to move unanimously in any direction.

“We hope to see progressives in lockstep with each other, but it’s not our place to tell Warren what to do,” Stamp told NBC News. “We hope we can signal some of the Warren coalition that they can join us."

The Working Families Party backed Sanders during his 2016 bid against Hillary Clinton, so their Warren endorsement last year led to some blowback online. For instance, the socialist magazine Jacobin, which supports Sanders, wrote at the time that the group had “written itself out of history.”

Stamp said the group would move past that, but acknowledged the online vitriol may dissuade some former Warren supporters from joining Sanders’ movement — something Warren herself warned against in her interview with Maddow.

“We're not going to act like the reaction to our endorsement was where we wanted it to be and what we wanted it to be," Stamp said. "And we’re not going to act like it did not hurt individuals at our organization or our organization as a whole. But we hope to show and model how to have dialogue and debate that’s principled.”