The Working Families Party and Justice Democrats endorsed Sen. Bernie Sanders for president ahead of Tuesday's contests in six states including Michigan, where Sanders hopes to score a surprise win over former Vice President Joe Biden.

"In a race where the stark contrast couldn't be clearer between Bernie and Biden, it's critical for progressives to stay in the fight," Working Families Party national director Maurice Mitchell told HuffPost Monday.

WFP initially endorsed Sen. Elizabeth Warren (D-Mass.) for president, but despite a brief time as a frontrunner, Warren's campaign was unable to draw support from voters in the primary and the senator bowed out on March 5. Warren has not yet made an endorsement in the primary race.

BREAKING: We’re endorsing @BernieSanders for president!



WFP supporters nationwide are mobilizing to help Bernie win so we can defeat Trump and build an America that works for the many, not the few. Sign up and let us know you’re in: https://t.co/MLDRrhIW7m 1/ pic.twitter.com/tqu3CBmfeG — Working Families Party (@WorkingFamilies) March 9, 2020

Mitchell, in his comments to HuffPost, said that while his group was disappointed in Warren's poor showing, the state of the race after her departure made clear who to support.

"When we endorsed Elizabeth Warren, we felt it was important to state the obvious: that there were two progressives in the race and it was actually a good thing that there were two bold progressives in the race arguing for structural change of our democracy and our economy," said Mitchell. "And now there's one progressive in the race. And we are still committed to that change—the change that we felt Elizabeth Warren's campaign was presenting a unique case for."

People's Action organization Citizen Action of New York director Rosemary Rivera welcomed WFP's endorsement and said it was just the latest example that the "multiracial working class is united behind Bernie Sanders."

"The Working Families Party has leveraged their power to put progressives in office all over the country," said Rivera. "They provided serious muscle on behalf of Elizabeth Warren's campaign and we at Citizen Action of New York supported them in that. Their endorsement of Sanders is game-changing, unifying, and healing."

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Justice Democrats on Sunday morning announced the group was throwing its support behind Sanders, with executive director Alexandra Rojas defining the Vermont senator as the remaining candidate with "the vision to defeat Trump and govern America in this time of crisis."

"There is no question that if Bernie Sanders had not run for president in 2016, organizations like Justice Democrats would not exist," said Rojas. "Over the past four years, he has galvanized a movement that made Medicare for All, a Green New Deal, free public college, ending mass incarceration and deportation, confronting skyrocketing inequality, and rejecting corporate money into mainstream positions in the Democratic Party."

We are so proud to endorse @BernieSanders for President and will be working to ensure we have candidates at every level focused on delivering relief to millions of working people across this country.



He has the vision to defeat Trump and govern America in this time of crisis. pic.twitter.com/wpbbtTnBff — Justice Democrats (@justicedems) March 8, 2020

In a statement, People's Action director of Movement Politics Ryan Greenwood said the stakes of the election are clear.

"The question for voters in the March 10 primary states is simple: Whose side are you on?" said Greenwood. "Joe Biden has been on the side of the credit card companies and NAFTA backers. Bernie Sanders is on the side of the multiracial working class."

Rep. Pramila Jayapal (D-Wash.), a Sanders surrogate, said that WFP's move was the correct decision.

"There is an important choice here before us and this is absolutely the right move," said Jayapal.