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The Working Families Party, the labor-aligned third party that originated in New York and has sought to become a national force, endorsed Senator Bernie Sanders of Vermont for president on Tuesday, handing him a symbolic boost from the state that twice elected Hillary Clinton to the Senate.

The party pledged that it would put its political muscle into helping Mr. Sanders win Democratic primaries, in a rigorous test of its impact beyond state and local politics. The decision to back him came after the party conducted an Internet poll, soliciting input from liberals nationwide.

Dan Cantor, one of the party’s founders, said in an interview that Mr. Sanders would help it “build the political revolution that will make possible the nation we want to see.”

But the endorsement of Mr. Sanders also proved divisive within the Working Families Party leadership, which includes a number of influential union leaders who support Mrs. Clinton. Mike McGuire, the political director of the Mason Tenders District Council of Greater New York and a Clinton supporter, said in an interview that he believed party officials had “rigged the system to make sure it went for Bernie.”

The party is also closely associated with top New York Democrats, including Mayor Bill de Blasio, who have already given their support to Mrs. Clinton’s campaign. A spokesman for Mr. de Blasio said the mayor had no role in the Working Families Party’s endorsement.

The Sanders campaign hailed the party’s endorsement as a sign of strength for the liberal lawmaker.

“Bernie has devoted his career to helping working families and is gratified by the support of the Working Families Party,” Mr. Sanders’s campaign manager, Jeff Weaver, said. “Together they will work to take on the billionaire class and change the rigged economy that is being held up by a corrupt political system.”

The precise impact of the endorsement remains to be seen. Mr. Sanders, who surged in national polls over the summer, remains a clear underdog in the Democratic race. While the Working Families Party has expanded into more than half a dozen states from its New York base, it does not operate in any of the states that hold early presidential primaries.

Still, supporters of Mr. Sanders said the endorsement was a significant moment, for both the candidate and for a party seeking to leave a deeper mark on national politics, particularly around issues of economic inequality.

Katrina vanden Heuvel, publisher of The Nation magazine and a prominent liberal activist, called the party’s support “the biggest endorsement for Sanders yet.”

“They have an active membership that can be deployed in campaigns, and they have savvy,” she said.

Zephyr Teachout, the law professor who challenged Gov. Andrew M. Cuomo in the Democratic primary last year, said Mr. Sanders lined up closely with the party on issues of economic fairness and campaign finance.

Ms. Teachout, who also endorsed Mr. Sanders on Tuesday, had sought the Working Families Party’s backing in her bid for governor, but the party endorsed Mr. Cuomo.

But some senior party members expressed stern disapproval of the process behind the endorsement.

In New York, leaders of the Working Families Party held a conference call in late November to announce plans for an online endorsement vote, startling some longtime party members who had expected direct input on the endorsement.

Over a hundred party leaders participated in the deliberations and voted in favor of sending the endorsement decision to an online ballot aimed at liberals across the country.

But several labor leaders and activists also voiced concern that an online poll would be heavily advantageous to Mr. Sanders, who has an energetic activist following.

Letitia James, the New York City public advocate and a supporter of Mrs. Clinton, said in a statement that she was “disappointed in their decision” to endorse Mr. Sanders.

Bertha Lewis, one of the party’s founders, said that the Internet ballot seemed like a “bait and switch” and that the Working Families Party had too often passed over female candidates to support white men like Mr. Sanders and Mr. Cuomo.

“I made it very clear I thought, all things considered, that we should endorse Hillary Clinton,” Ms. Lewis said. “I thought the most revolutionary act that we could possibly do was to finally put a woman in the White House.”

Ultimately, Mr. Sanders won 87 percent of the votes in the online poll, according to two party officials.