The endorsement comes at a critical moment for Sanders (I-Vt.), who risks falling significantly behind former vice president Joe Biden in Tuesday’s primaries, which include the pivotal state of Michigan. In recent days, the Sanders campaign has been attacking Biden on Social Security and other issues and is working hard to energize liberal groups.

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Warren herself did not back Sanders since dropping out of the race last week. Mitchell refrained from urging her to do so, and Rep. Alexandria Ocasio-Cortez (N.Y.), a leader of the Democratic Party’s liberal faction and a Sanders supporter, also urged caution.

“There are folks telling Warren what to do now, when they weren’t in her camp,” Ocasio-Cortez said in an interview. “It’s not up to me to tell her what to do. What I do know is that if we want to win Medicare-for-all, we have to build overall, beyond electoral politics. We have to build a broad, progressive coalition.”

Some of Sanders’s supporters are angry at Warren, who like Sanders is a leader of the party’s liberal wing, because of her disputes with Sanders and her failure to immediately endorse him when she dropped out — while a parade of centrist former candidates are rallying behind Biden.

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But Ocasio-Cortez warned against such finger-pointing. “When things start falling short, I don’t think that seeking out someone to blame, instead of figuring out how to adapt, is the smart move,” she said.

The WFP had endorsed Warren in September after its members and its leadership took a vote; even then, said Mitchell, Sanders had been the most popular second choice. The group has affiliates in Wisconsin, New York, Pennsylvania and Connecticut, all states with April primaries, and Mitchell said they’d be ready to “play the delegate game” and help amass support for Sanders by tapping their thousands of supporters on the ground.

The WFP is the latest of several pro-Warren groups that have gotten behind Sanders since the senator from Massachusetts ended her campaign. The Progressive Change Campaign Committee, which has spent nearly a decade working with Warren, has urged supporters to vote for Sanders in the upcoming Michigan primary. Justice Democrats, a group founded by Sanders organizers, had been neutral while Warren was in the race; on Sunday, it endorsed Sanders.

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Left-leaning and liberal groups that did not endorse Sanders earlier in the campaign had faced criticism on the left, and the WFP was no exception. When it backed Warren, critics accused the party’s leadership of skewing away from membership, pointing out that the votes of rank-and-file WFP supporters counted less than the vote of board members. The WFP did not release a vote count, as it had in 2016, when it endorsed Sanders over Hillary Clinton.

Warren did not forget that. In an interview last week with MSNBC’s Rachel Maddow, Warren criticized Sanders’s most zealous supporters for attacking groups that backed her or had remained neutral, singling out the WFP.

“Working Families Party, two women there, women of color, who were attacked right after they endorsed me,” Warren said. “We are responsible for the people who claim to be our supporters and do really threatening ugly dangerous things to other candidates.”

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Mitchell defended the WFP’s support of Warren, which by last month had made it an outlier among grass-roots left-leaning organizations.

“Warren ran a historic progressive campaign that gave the knockout blow to an oligarch who was attempting to buy the election,” he said, referring to the senator’s commanding debate performance against former New York mayor Mike Bloomberg. “She set the bar for how presidential campaigns can work in collaboration with social movements.”

Ocasio-Cortez, who also missed out on the WFP’s endorsement during her 2018 run for Congress, defended the party’s process, saying it was not designed to favor any particular candidate. Ocasio-Cortez endorsed Sanders in October and is credited with giving fresh energy to his campaign at a crucial moment after he had suffered a heart attack.