A group of far-left activists huddled in the basement of a labor union in Manhattan, aiming to upend a Democratic institution that they felt had grown stale.
The potential target was not an entrenched politician, or the local county party. It was a much closer ally: labor unions, including the one that was hosting the activists’ meeting earlier this year.
The plan did not go over well. The union, a branch of the Communications Workers of America, kicked the activists out. Labor leaders accused the activists of plotting infiltration. The activists, in turn, recently warned of union spies.
The dispute makes clear the growing ambition of New York’s activist left, which over the past year has notched a string of high-profile successes, from propelling Representative Alexandria Ocasio-Cortez’s election to scuttling Amazon’s plans to build headquarters in New York City.
On the national stage, progressive forces are trying to push the Democratic agenda to the left, and challenge how party leaders respond to President Trump.
Emboldened by their successes, they are now preparing to take additional steps toward political and economic transformation — including by challenging groups that have long been in the populist vanguard of the Democratic Party.
The unions, in turn, have been forced to reckon with the shifting political landscape, and to decide what position they want to occupy within it.
While unions in New York and nationwide have often championed progressive policies, some have also opposed marquee issues for the left, including the Green New Deal and Medicare for All. No union endorsed Ms. Ocasio-Cortez in her insurgent congressional campaign last year.
Even as grass-roots labor activism has swelled in new sectors — including among video game makers and Uber drivers — established unions have often kept their distance because of funding shortages or reluctance to engage with workers outside of their formal membership.
At its heart, the debate is one between pragmatism and idealism, working within the system versus burning it down.
It is the same debate dogging the Democratic Party at large, but amplified by an only-in-New-York mix of vibrant activism, impenetrably blue politics and — unlike in the rest of the country — still-mighty unions eager to quell perceived threats to their clout.
“Given the political moment we find ourselves in, the idea that a leftist political organization would launch disruptive attacks on their ostensible allies in the labor movement is the definition of cutting off your nose to spite your face,” said Peter Ward, the president of the Hotel Trades Council, a union of hotel workers.
Activists countered that the labor movement had fallen out of step. Jeremy Saunders, the co-executive director of VOCAL-NY, a progressive activism group, said many union leaders seemed dismissive of the energy on the left.
“This growing left sees labor as its natural allies,” he said, “but is frustrated by some unions who too often side with those same politics and politicians who refuse to fight for justice.”
The current dispute involves the New York City branch of the Democratic Socialists of America and the plan it crafted at that basement meeting. The socialists, deeming the city’s unions overly reliant on insider relationships, prepared a “rank-and-file strategy” for their members to join unions and remake them from within.
The group also later published a blog post encouraging other chapters nationwide to follow suit.
Some backlash was immediate. But it exploded recently, after the group’s 37-page memo about its plan was reported by Politico, leading union leaders to accuse D.S.A. of sowing division.
In response, D.S.A. members — including State Senator Julia Salazar, the first member of the group to serve in the Legislature — said the union leaders were “red-baiting.”
Tensions escalated when D.S.A. issued a statement accusing the Hotel Trades Council of sending members to spy on D.S.A. meetings in order to “thwart H.T.C. members’ self-organization and full exercise of their democratic union rights.” (The union denied spying; D.S.A. meetings are public.)
In interviews, activists — both involved with D.S.A. and not — said the unions’ reactions affirmed their critique.
Bianca Cunningham, a leader of the New York City D.S.A., said unions’ entanglement in political relationships “prohibits them from being able to take real chances.”
“We’re not surprised that conservative unions come out and say they don’t support this kind of strategy,” she said, “because they see it as a threat to their own power.”
To be sure, the labor movement is large and diverse. Some unions have aggressively backed progressive priorities. Activists and organized labor in New York have successfully pushed together for a $15 minimum wage and driver’s licenses for undocumented immigrants. D.S.A. still borrows meeting space from some unions.
Attempts to remake the unions are also not limited to outside activists: Internal factions within several unions have publicly challenged leadership.
Still, even avowedly left-leaning labor leaders bristled at what they perceived as the activists’ naïveté or arrogance.
John Samuelsen, the international president of the Transport Workers Union, said activists were “welcome to join the fight,” adding that he endorsed Senator Bernie Sanders for president in 2016. But he emphasized the historical power of unions, pointing to Gov. Andrew M. Cuomo’s leftward tilt in recent years.
“It wasn’t the D.S.A. that did that. The D.S.A. is incapable of doing that,” Mr. Samuelsen said. “It’s the institution of organized labor in New York that drove Cuomo to the left.”
Michael Mulgrew, the head of the city’s teacher union, said the activists’ disillusionment was misplaced.
“Our track record on being progressive is pretty solid to say the least,” he said, adding, “You don’t need to push us. We’re pretty far out there in terms of how we’re trying to change society in terms of the betterment of all.”
Even Ms. Salazar said she understood the unions’ defensiveness: “There’s a little bit of, ‘We’ve been doing this for a long time. You just got here.’”
The skirmish in many ways encapsulates the challenges left-wing activists face as they strive to take their message mainstream.
They have struggled to shed the label of interloper: Many activists only turned to politics after the 2016 presidential election. D.S.A. in particular has seen membership skyrocket, from 5,000 in 2016 to more than 50,000.
The activists have also often been painted as white and well-off — a fact even D.S.A. acknowledged in its memo, writing, “D.S.A. has described itself as committed to maintaining an active and diverse membership but is primarily composed of middle-class white people.”
Mr. Ward, of the hotel union, seemed to allude to that when he said that his union was “made up mostly of women, immigrants and people of color,” who would meet D.S.A. with “very aggressive and overwhelming pushback” if the activists did not share their goals.
But the rift also underscores how influential the newcomers are becoming.
Grass-roots activist groups have shown that they can run formidable get-out-the-vote operations, akin to those that make unions such appealing political allies. D.S.A. volunteers played a key role in Ms. Ocasio-Cortez’s primary victory, as well as in the candidacy of Tiffany Cabán, a former public defender who last month lost her bid for Queens district attorney by just 55 votes, despite earning just one labor backer.
Labor leaders will need to adapt, said Shaun Richman, the program director at SUNY Empire State College’s center for labor studies. He said he expected to see unions taking more political risks and endorsing more challengers.
“I think there’s a feeling among labor leaders that they want to be seen as, ‘They will take out a lackluster incumbent when the opportunity arises,’” Mr. Richman said. “That’s the pathway to maintaining some power and influence within the modern Democratic Party.”
The activists, for their part, said they wanted to strengthen, not undermine, their alliance with organized labor.
“There have been clear and obvious moments when we have been on different sides,” Cea Weaver, a member of the New York City D.S.A. steering committee, said.
But, she continued, “Labor has to be part of a winning strategy for the left. So the question is, what will it take?”