(CNN) The Working Families Party endorsed Sen. Bernie Sanders on Monday, less than a week after its first choice, Sen. Elizabeth Warren, dropped out the Democratic presidential race .

The announcement comes on the eve of another round of crucial Tuesday primaries, including a do-or-die contest in Michigan , a state that Sanders won in 2016 and Democrats are desperate to reclaim from President Donald Trump this November. WFP leadership decided to back Sanders -- and make the argument to Warren loyalists that they should do the same -- after it became clear that the nominating contest would come down to either him or former Vice President Joe Biden.

"We understand that there are people on the fence. We are, just like them, Warren supporters who are disappointed that our candidate left the race, but there's work that remains to be done and we're going to be committed to the change that brought us to the Warren campaign," WFP national director Maurice Mitchell said. "Sen. Sanders' campaign is a natural home for Warren forces that are looking to get the job done."

The WFP, which has expanded in size and influence since the 2016 election, endorsed Warren last September after backing Sanders' primary bid four years ago. That decision set off an angry backlash among some of the Vermont senator's supporters, who threatened and harassed the group's officials on social media. Warren recalled the episode during an interview last week and suggested Sanders had not done enough to curtail similar outbursts.

"The candidates are responsible for their supporters, and all candidates had supporters who got out of line," Nelini Stamp, WFP's director of strategy and partnerships, told CNN. The decision to back Sanders, she said, was a pragmatic one: He finished second in their first round of ranked-choice voting last year and largely shares a platform with Warren.

"I actually don't think those people have power," Stamp said of the Sanders supporters who attacked her online. "But the reality is that they're loud and it hurts. I'm not going to lie (and say that) it doesn't hurt. But I think that we can create a new model of a center of gravity."

Warren has not publicly indicated whether she plans to endorse Sanders, Biden or remain on the sidelines until the nomination is decided. She joked during an appearance on "Saturday Night Live" this weekend that she might "pull a New York Times and endorse them both," a nod to the outlet endorsing both her and Minnesota Sen. Amy Klobuchar, who has also since dropped out of the race. For now, Warren appears mostly likely to stay out of what could turn into an ugly run-in to this summer's Democratic convention.

"I would love to continue to be in the fight with Sen. Warren. We've worked with her for many years and I would love to continue seeing her fight for the issues that are important to her," Stamp said. "But she should take her time. She should do what's right for herself, as well."

The WFP joins Justice Democrats, the organization that recruited New York Rep. Alexandria Ocasio-Cortez ahead of her 2018 run, in endorsing Sanders. Justice Democrats made their choice official on Sunday morning after staying neutral over the past 14 months. Both groups are trying to bring Warren's supporters around to Sanders, but it remains unclear how her base will break on Tuesday and beyond.

Warren won the WFP's endorsement late last summer at what would turn out to be a high point in her campaign. Five days after their endorsement landed, a CNN/Des Moines Register/Mediacom poll showed the Massachusetts senator leading the Democratic field in Iowa -- ahead of Biden and picking up steam at Sanders' expense.

But Warren's fortunes began to fade in the fall, when the debate over "Medicare for All" pushed her into a political no-man's land between the party's progressive and moderate factions. Biden, Klobuchar and former South Bend, Indiana, Mayor Pete Buttigieg attacked her from the right, while some on the left questioned Warren's plan to effectively break up the legislation into two pieces, a charge that damaged her standing with progressive voters.

In addition to the online abuse aimed at its leaders, the WFP, a third party that got its start two decades ago in New York and has helped elect progressive down-ballot candidates around the country, also faced sharp criticism from reputable leftist voices over its decision to back Warren.

The socialist magazine Jacobin reacted to news of the party's choice by declaring, "The Working Families Party Has Written Itself Out of History" and argued that organization's "opaque presidential endorsement process signaled a rejection of not only Bernie Sanders but the movement emerging around him."

The hard feelings also traced back to 2016, when Sanders, after endorsing Hillary Clinton, urged supporters before the general election to vote for the former secretary of state on the WFP ballot line. The party, he said then, represented "the closest thing there is to a political party that believes in my vision of democratic socialism."

In a statement last September, when the WFP announced it was supporting Warren this time around, Mitchell praised both of the progressive leaders.

"Senator Warren and Senator Sanders have both shaped the ideological terrain on which this campaign is being waged," he said at the time. "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."

But after a campaign that tested the decades-long alliance between Warren and Sanders, that "partnership" appears to be in serious doubt.

The Progressive Change Campaign Committee, the group most closely aligned with Warren, has stopped short of issuing an outright endorsement. Instead, it sent an email to Michigan Democrats over the weekend urging them to vote for him "strategically" in order "to stop the premature ending of this primary election."

"Elizabeth Warren and Bernie Sanders share many goals. If you love Bernie Sanders and his agenda, that's the best reason to vote for him," the PCCC wrote to supporters. "And if you are a die-hard Joe Biden supporter, by all means vote your conscience."