Last night Mayor-elect Bill de Blasio was again the darling of the left.

Headlining a Brooklyn fund-raiser for the group New York Communities for Change, Mr. de Blasio was hailed as a progressive hero and the fruit of a more than decade-long battle by labor groups, grassroots organizations and the Working Families Party to crown one of their own.

“I am so appreciative of not just the support I received and my campaign received, I’m appreciative of all the people who were given hope by New York Communities for Change, who were shown their own power,” said Mr. de Blasio, speaking at the Transport Workers Union Local 100’s headquarters in Brooklyn.

The fund-raiser drew hundreds of attendees, including many of the city’s political elite, allowing for the awkward collision of the Democratic powers-that-be and the strident, anti-establishment rhetoric of some of the event’s speech-makers. Chris Shelton, a vice president with the Communication Workers of America, urged the attendees noshing on brie cheese to join a “revolution” against the “bankers, billionaire and brokers of Wall Street.”

In particular, the fund-raiser honored the late Jon Kest, a founder NYCC who also helped launch the WFP, and served as a coming-out party for the city’s many liberal activists feeling emboldened by Mr. de Blasio’s ascension. Staunch de Blasio allies like George Gresham, the president of the powerful healthcare workers’ union, and Sex and the City actress Cynthia Nixon were honored, too, as well as Mr. de Blasio himself.

“We are in a progressive moment right now,” declared East Harlem Councilwoman Melissa Mark-Viverito, an early endorser of Mr. de Blasio and a leading candidate for council speaker. “This honoree [Mr. de Blasio] represents the culmination of Jon’s tireless, brilliant and visionary work to build our city’s progressive infrastructure and to build coalitions across groups that made our movement stronger … It was this infrastructure that was built that just about a month ago enabled our city to elect its first progressive Democratic mayor in 20 years, our Mayor-elect Bill de Blasio.”

There were also political subtexts: Mr. de Blasio hugged Ms. Mark-Viverito as he took to the podium in the drab, sprawling conference room. Insiders have long speculated that Ms. Mark-Viverito, a liberal firebrand, is Mr. de Blasio’s favored candidate for the city’s second most powerful post–and Mr. de Blasio added fuel to the fire in his remarks

At one point, he heralded her as a “wonderful, progressive visionary” and noted that she was the first sitting council member to endorse him “and that was in the lean times before things got quite so interesting. I am deeply appreciative of her commitment to progressive causes and all she’s done for this city.”

Still, Mr. de Blasio reserved much of his praise for NYCC, a community organizing group once known as the Association of Community Organizations for Reform Now, or ACORN. ACORN, which fought for liberal causes like raising the minimum wage and ending predatory lending, disbanded several years ago after an embarrassing scandal dried up its funding streams.

NYCC lent Mr. de Blasio’s mayoral campaign organizing might and is now hoping to revolve in his new orbit of power.

Despite the assertion of NYCC’s clout, however, there were hints of the realities Mr. de Blasio must reckon with as he straddles the worlds of the left and Democratic establishment. Ms. Nixon, tongue-in-cheek, warned Mr. de Blasio that the city’s left-wing institutions would hold him accountable, even if he was their current darling.

“NYCC will never hesitate to speak truth to power,” she said, “and to check our new mayor if suddenly he goes rogue and starts spending his days having three-martini lunches with Wall Streeters at Cipriani’s and his nights riding around with Chirlane through moonlit Central Park in one of those horse carriages he was supposed to ban.”

Ms. Nixon failed to note, however, that Mr. de Blasio already breakfasted with a leading hedge fund manager and investment banker–part of a larger effort to entice the city’s business elite to accept his agenda.

Correction (12:38 p.m.): An earlier version of this post incorrectly stated Bertha Lewis as a head of NYCC. She is best known for leading the now-defunct community organizing group ACORN.