2 of New York’s Most Influential Offices Are About to be Held by One Person

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Corey Johnson, in his first year as speaker of the New York City Council, has gotten good at making Mayor Bill de Blasio uncomfortable.

There was the grilling that Mr. Johnson and the Council gave Amazon, after Mr. de Blasio helped lure the company to Long Island City, Queens. The Council has eagerly introduced bills the mayor opposes — ranging from legislation that would legalize electric bicycles to creating a kind of commercial rent regulation. Mr. Johnson has also frequently suggested the mayor take over the faltering state-run subway system whose failures Mr. de Blasio has preferred to blame on Gov. Andrew M. Cuomo, who controls the Metropolitan Transportation Authority.

Now, Mr. Johnson is about to take a job that was tailor-made to cause even more agita in City Hall: public advocate.

By a quirk of city law, Mr. Johnson will become the acting public advocate — next in the line of succession to the mayor — at midnight on Jan. 1.

And Mr. Johnson, whose obvious ambition for higher office causes some to doubt his motives, is not about to let the opportunity of the extra bully pulpit pass him by.

“Everything I do, I do in a full-throated, full-hearted way and I want to take this role seriously,” he said in an interview.

He will hold the dual posts until Feb. 26, when a special election will take place to replace the outgoing public advocate, Letitia James, who was elected state attorney general in November and will begin her new job on Jan. 1. Mr. Johnson, who will continue as the Council’s speaker as he moonlights as public advocate, is planning an aggressive schedule to take full advantage of his twin roles.

Donning his public advocate hat, he plans to call attention to the performance of the city’s 311 complaint line by releasing data about response times at various city agencies, and the number of calls it takes to get a problem corrected. Then, as City Council speaker, he will hold hearings about the 311 data.

He said he plans to add interactive maps, created by the Council’s data team, to the public advocate’s website to help pair students with schools. He has scheduled a survey of subway riders — online and in person — and will be making appearances at train stations in all five boroughs to query passengers over five days in January.

The subject of mayoral control of the subways, he said, may come up. Mr. Johnson, in an interview, compared the idea to former Mayor Michael R. Bloomberg’s successful effort to control the city’s public schools in 2002.

Eric F. Phillips, the mayor’s press secretary, said the proposal was nothing new. “Lots of people have fantasized over it through the years — and with good reason,” he said. “The mayor doesn’t oppose the conversation, he’s just more focused on things that will help the subways now.”

Mr. Johnson’s eagerness to seize his additional clout is perhaps no surprise: For much of his first year as Council speaker, he has unabashedly embraced the spotlight while leading a body that is itching to assert its power vis-à-vis the mayor.

A recent cover of City and State, a New York political magazine, asked the pointed question: “Is Corey Johnson Already Mayor?” (Aides to the mayor chafed at the headline.)

Mr. Phillips dismissed the attention Mr. Johnson’s temporary role is likely to garner. “We’ve got a big city to run and I don’t think anyone paid any attention to it, to be honest,” Mr. Phillips wrote in an email, referring to the article in City and State. “But it sounds like a great story for him.”

As leader of the Council, Mr. Johnson has ushered in new laws to subsidize subway rides for poor New Yorkers, cap for-hire vehicles like Uber and force Airbnb to make more information available to the city to curtail illegal rentals.

Mr. Johnson’s ubiquity and energy have defined his public persona. It is a contrast to Mr. de Blasio, whose approach, observers said, can appear more cerebral and sometimes distant; recently, he has been spending less time at City Hall.

Mr. Johnson, who dodges questions about a mayoral run, is arranging for semiregular “Tuesdays with Corey” segments on Fox’s local morning show next year, an echo of NY 1’s “Mondays with the Mayor” interviews.

“They are both progressives, but stylistically they’re very different,” said Joseph P. Viteritti, a professor of public policy at Hunter College who wrote a book about Mr. de Blasio. “Corey is a rough-and-tumble street politician. De Blasio is intellectual about things. He’s not comfortable with the press. He’s not a glad hander. It’s a very different style than Corey’s.”

Mr. Johnson is the first person to serve temporarily as public advocate, a citywide elected office that was first created after the city’s 1989 charter revision.

“This is a kind of rare bird,” Mark J. Green, the first to fill the public advocate role, said of Mr. Johnson’s abbreviated tenure. “It’s sort of like him putting on a costume for New Year’s Eve or Halloween and trying it on for size.”

The job affords little direct power but has been a launching pad to higher office: Four people have been elected to the job since 1993, including Ms. James and, just before her, Mr. de Blasio.

For that reason, more than a dozen declared and all-but-declared candidates will be running in the nonpartisan special election in late February.

“The role of the public advocate is to be the ombudsman to the city,” said Betsy Gotbaum, who served as the city’s second public advocate from 2002 to 2009. She said the job can be challenging. “It’s a tiny little office with no budget,” she said.

Mr. Johnson also said he will take up an issue highlighted by Ms. James: the city’s little-known Commission on Public Information and Communication. He will call for the commission to create a directory of all data sets that agencies maintain to help ensure compliance with the city’s open data law.

The move could meet resistance from City Hall. While the Law Department in 2014 determined that the city’s open data portal met the City Charter’s requirement for a public data directory, “there is a lot of information missing,” said Councilman Ben Kallos, the Council’s appointee to the commission.

“With the support of a speaker like Corey Johnson, who is interested in using every tool he’s got to stand up to the mayor and who will soon be our next public advocate, I look forward to what can be accomplished in 45 days and thereafter,” Mr. Kallos said.

As for the more contentious issue of city control over the subway system, Mr. Johnson hinted that he had a plan, and that it would likely be unveiled within the next two months.

“I’m going to soon have more to say in a very detailed way about what we think that should look like,” he said. “But I think it’s a conversation that we have to start having now if we want to accomplish it, hopefully a couple of years from now.”

He added: “Stay tuned!”