The Working Families Party, the labor-aligned progressive group whose electoral influence has grown since the 2016 election, has endorsed Senator Elizabeth Warren of Massachusetts for the Democratic presidential nomination, a boon to her candidacy as she attempts to position herself as the main challenger to former Vice President Joseph R. Biden Jr.
The party endorsed Senator Bernie Sanders of Vermont during the last presidential cycle, at which time he described Working Families as “the closest thing” to “my vision of democratic socialism.” The group’s endorsement of Ms. Warren on Monday, one of the few by a prominent progressive organization this early in the primary, could turn heads among left-leaning Democrats desperate to defeat Mr. Biden, the more moderate front-runner, in a primary election where their party’s ideological future is at stake.
“Senator Warren knows how to kick Wall Street kleptocrats where it hurts, and she’s got some truly visionary plans to make this country work for the many,” said Maurice Mitchell, the Working Families Party’s national director. “We need a mass movement to make her plans a reality, and we’re going to be a part of that work.”
Mr. Mitchell brushed off the possibility that the group’s endorsement would be seen as a sign of a splintering of the progressive left. The vote among “tens of thousands” of party members and national committee leaders resulted in a commanding majority for Ms. Warren, a party spokesman said; she received more than 60 percent of the votes on the first ballot.
But the announcement was met with derision from some of Mr. Sanders’s supporters. The national committee leaders — 56 people — held 50 percent of the voting power, with party members accounting for the other 50 percent. Several Sanders supporters called for the Working Families Party to release the full vote totals, which it has declined to do.
Mr. Sanders’s campaign shook up its New Hampshire staff over the weekend as Ms. Warren continued to make inroads among progressives. Ms. Warren is coming off a debate performance last week that was generally well received, and she held a rally in New York on Monday that drew thousands to Washington Square Park.
Also on Monday, Ms. Warren unveiled a plan to combat corruption in government, a core theme of her campaign. The plan is based on a wide-ranging anticorruption bill that she first proposed last year and is a cornerstone of her stump speech on the campaign trail.
Mr. Mitchell and other Working Families Party leaders said in interviews that their endorsement came with a message to other progressive organizations. Rather than passively observe the primary, they said, these groups should choose a side and flex their organizing muscle during the early stages to help knock Mr. Biden off his perch.
“If our focus is on victory, we can’t be delusional about it,” Mr. Mitchell said. “You don’t defeat the moderate wing of Democrats through thought pieces or pithy tweets, you defeat their politics through organizing.”
Traditional bellwether endorsements from labor unions like American Federation of Teachers and the Service Employees International Union have not yet materialized, though Mr. Sanders picked up an endorsement from the United Electrical workers in August. With less than five months to go before the Iowa caucuses formally begin the nominating contest, many organizations are still wrestling with a sprawling Democratic field.
“There are some great arguments for progressives rallying around a single candidate as soon as possible and others for taking the time to see how the contest develops,” said Yvette Simpson, chief executive of Democracy for America, another progressive group that endorsed Mr. Sanders four years ago. Ms. Simpson said her group did not plan to endorse a candidate before December.
Mr. Sanders finished second in the Working Families Party vote, ahead of three other candidates: the former housing secretary Julián Castro, Senator Cory Booker of New Jersey and Mayor Bill de Blasio of New York.
Most national polls show a top tier of three candidates in the Democratic primary — Mr. Biden, Ms. Warren and Mr. Sanders — and Mr. Biden has remained the front-runner.
Among the two leading progressive candidates, Mr. Sanders has traditionally enjoyed more support from working-class voters, helped by his name recognition held over from 2016. Ms. Warren has made significant gains in the last six months, but polling shows that most of that growth has come from college-educated voters.
In the years since President Trump’s election, candidates backed by the Working Families Party have won congressional, state and local races across the country, expanding the group’s power base from the Acela corridor to the West Coast. The group also helped several left-wing candidates win seats in the New York State Legislature last year.
Mr. Mitchell took over as national director for the party in 2018, after he rose in prominence during the Black Lives Matter movement born out of protests in Ferguson, Mo.
The group’s leaders stressed that, even with their endorsement, their intention was not to divide the progressive left between Ms. Warren and Mr. Sanders. The senators are longtime friends and have been publicly supportive of each other’s candidacies.
But they are still locked in a battle to be the progressive standard-bearer. After the Working Families endorsement was announced on Monday, Misty Rebik, Mr. Sanders’s Iowa state director, wrote on Twitter: “There is one movement politics presidential candidate. One. It’s @BernieSanders.”
Outside the left wing of the party, more moderate Democrats have continued to sound alarms about progressives’ ability to beat Mr. Trump, and about Ms. Warren’s chances in particular. And many Republicans see the progressives’ embrace of a “Medicare for all” health care system and the Green New Deal as general-election liabilities waiting to be exploited.
Surrogates for other Democratic contenders — like Mr. Biden, Ms. Harris and Mayor Pete Buttigieg of South Bend, Ind. — have tried to paint Ms. Warren and Mr. Sanders as candidates of the elite, who talk about lofty ideologies instead of pragmatic solutions.
“My concern about the vision from the Sanders-Warren approach is that it can polarize Americans, when we have other ways to deliver bold solutions without dividing the American people further,” Mr. Buttigieg said Sunday on CNN.
But Mr. Mitchell said he believed the Working Families Party could convert Democrats skeptical of wide-reaching progressive policies.
“I’m not worried about converting people who are already committed to a structural change agenda,” Mr. Mitchell said. “I’m worried about the people who are still trying to figure out where they land.”
Thomas Kaplan contributed reporting.