Brad Lander, a cerebral and unassuming city councilman from Brooklyn with a degree in anthropology, has abruptly become the sort of person who endlessly fascinates scholars of the modern metropolis: a power broker.

Allies greet him with mazel tovs and hugs. Supporters ask if he will run for mayor in 2021. An old friend, spotting him at a social gathering, bows deeply — a not entirely facetious acknowledgment of Mr. Lander’s new status as a kingmaker in City Hall.

A former housing advocate who succeeded one Bill de Blasio as a Democratic councilman from Park Slope four years ago, Mr. Lander and another passionate liberal, Melissa Mark-Viverito, joined with several other lawmakers, some of whom had been backed by the Working Families Party, to form a Progressive Caucus.

They pushed to improve pay and benefits for low-wage workers. They argued for letting city residents help decide how lawmakers should spend their discretionary funds. And in 2011, they began making endorsements and bundling money for candidates, hoping to increase their ranks and perhaps play a role in choosing the next speaker.

On Jan. 8, when Ms. Mark-Viverito was elected speaker, Mr. Lander hovered at her side, whispering in her ear — and confirming, in the eyes of several other legislators, the influence that he will now wield.

By Wednesday, when Mr. Lander presided for the first time over the Council’s Rules Committee — announcing coveted leadership positions and committee assignments, as well as his own new title, deputy leader for policy — some lawmakers were privately referring to him as a “shadow speaker.”

Mr. Lander, 44, seems to enjoy being at the center of the Council’s solar system, after so much time in outer orbit. In the space of a few hours one recent afternoon, he was on the phone nearly continuously, chatting with the Brooklyn Public Library president about appointing a new representative to its board; arguing with an editorial writer over Ms. Mark-Viverito’s plan to reform, but not abolish, member items; and assuring a supporter that promised reforms in the Council would be enacted swiftly.

That he can act decisively is not in question. In December, at the height of the battle for speaker, Progressive Caucus members awaiting a meeting where they expected to take turns arguing how to proceed were suddenly told the meeting was off: Mr. Lander wanted them to sign their names to a news release endorsing Ms. Mark-Viverito.

Some who clashed with Mr. Lander saw it as a disturbing sign that the Progressives’ ideals about collaboration, democracy and transparency would easily be set aside in pursuit of policy goals.

But Mr. Lander said he had acted on basic principles of organizing.

“You’re building a team of people that share goals and values,” he said in an interview. “You want to be collaborative and inclusive and democratic, but in good organizing, people also want to win. They want to achieve a concrete and specific change that requires smart strategy and the ability to move quickly at the right moment.”

If the caucus had not been acting swiftly at that point, he said, “We might well have lost.”

Liberal advocates who had worked with him said they were electrified by Mr. Lander’s elevation.

“It’s a thrill now that he’s in a position to really have a pretty big impact. It’s really exciting,” said Deborah Axt, co-executive director of Make the Road New York, which works on behalf of immigrants. After lobbying him to focus on the plight of renters affected by Hurricane Sandy, she mused that Mr. Lander, though still as low-key and approachable as ever, “has now become a player in this city in a way that he wasn’t before.”

Mr. Lander came by his political convictions early. Growing up in St. Louis in the 1970s, he idolized the Rev. Dr. Martin Luther King Jr. and Abraham Joshua Heschel, a rabbi who had been active in the civil rights and antiwar movements. As a teenager, he helped organize a march in Washington in solidarity with Jews trying to flee the Soviet Union. While studying at the University of Chicago, he attended meetings of the Democratic Socialists of America and painted public housing units on Chicago’s South Side.

He got a master’s degree in anthropology at University College London; followed his girlfriend, Meg Barnette, to New York (they later married); and wound up running the Fifth Avenue Committee, a community development organization in Park Slope that builds low-income housing and provides job training. In 2003, he left to head the Pratt Center for Community Development, which advises community groups on planning, housing and economic development.

By 2009, when Mr. de Blasio ran for public advocate, Mr. Lander had forged close ties with labor unions, the grass-roots organization Acorn and the labor- and Acorn-backed Working Families Party, whose endorsement carried him into office.

On the Council, Mr. Lander earned good will in part through his willingness to let others have a bigger share of the spotlight.

Late last year, Mr. Lander chose not to run for speaker himself, because, he said, someone had to focus on holding the Progressive Caucus together if it was to be a potent force in the leadership fight.

Linda Sarsour, a friend of Mr. Lander’s who is the executive director of the Arab American Association of New York, said she believed that Mr. Lander also thought it was important to elect the Puerto Rican-born Ms. Mark-Viverito as speaker, especially with white men in both the mayor’s and comptroller’s offices.

“He’s very conscious of being white, and he’s very conscious of being male and conscious of his privilege, which is something that you don’t really see often,” Ms. Sarsour said. “I really believe that he wanted to see a first Latina in that spot, and I think he wanted to be seen as someone who had something to do with that.”

Ms. Sarsour, who said she hoped Mr. Lander would run for Congress one day, said he had gained a host of admirers for speaking out against police surveillance of Muslims.

But his sensitivity did not end there, she said. In the fall, Mr. Lander’s daughter, Rosa, 10, was organizing a read-athon at the Park Slope Library to raise money for Afghan women, and he asked Ms. Sarsour to send a young colleague to talk to the children, mostly girls, so that they could hear from an educated and outspoken Afghan woman about why she chose to wear a hijab.

“This is how he thinks, just very conscious of, ‘Who am I to speak on this issue? I’m a white man speaking on young Afghan girls, or the place of Muslim women in another part of the world,’ ” Ms. Sarsour said.

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