Bill de Blasio, the former mayor of New York City, announced on Friday that he would run for Congress in a newly created district stretching from Lower Manhattan to his home in Brooklyn, testing a political return in an already crowded Democratic primary.

Mr. de Blasio, who left office with low approval ratings in December after two terms, has spent months openly mulling his future. He considered runs for governor and another congressional seat, and recently hired a public relations firm to help him offer his services as an on-call political commentator.

But when a state court released a slate of proposed congressional districts this week, unexpectedly creating a new safely Democratic seat in the heart of New York City, he saw an opening and seized it.

“This is a unicorn: a brand-new congressional seat that no one has ever seen before that has no incumbent,” Mr. de Blasio said in an interview on Friday, adding that he thought its voters might want “someone with some history and stature” at a moment of national tumult.

Last month, New York’s highest court struck down as unconstitutional Democrat-friendly maps drawn by the State Legislature and ordered a neutral expert to quickly draft replacements.

The expert, Jonathan R. Cervas, redrew the 10th Congressional District, which is currently held by Representative Jerrold Nadler, to include large swaths of Brownstone Brooklyn, the Orthodox Jewish community in Borough Park, as well as the East and West Village in Manhattan.

After eight years as mayor and a disastrous run for president in 2020, Mr. de Blasio will enter the race better known than almost any potential opponent, with both a record of progressive accomplishments and a trail of political disappointments.

He also has deep ties to important parts of the new 10th District, which includes his home in Park Slope, Brooklyn, a famously liberal neighborhood. Mr. de Blasio, 61, is an N.Y.U. graduate who got married in Prospect Park, and, as mayor, famously trekked from Gracie Mansion to his former neighborhood for late-morning workouts and stops at a favored patisserie.

“It’s a very special thing,” he said, “and happens to be all of the places that have been the core of my life.”

Assemblywoman Rodneyse Bichotte Hermelyn, the chairwoman of the Brooklyn Democratic Party, who is facing a progressive insurgency within her own borough, said that Mr. de Blasio was “the most qualified progressive candidate who I believe can win this diverse seat.”

But it is far from clear that New Yorkers, many of whom eagerly welcomed Mr. de Blasio’s exit from City Hall, are ready to support a comeback, particularly when so many other Democrats carrying less baggage have already shown interest in running for the seat in an Aug. 23 primary.

Early Saturday, after the final version of the district map was released, Representative Mondaire Jones, who currently holds a seat in suburban Westchester County, entered the 10th District race.

Mr. Jones had also been considering two other districts that would have pitted him in a looming primary fight with one of two neighboring congressmen, Jamaal Bowman or Sean Patrick Maloney.

Others who have expressed interest include State Senator Brad Hoylman, a Manhattan progressive; State Senator Simcha Felder, a conservative Democrat whose district includes Borough Park; and Assembly members Robert Carroll, Jo Anne Simon and Yuh-Line Niou, who was scheduled to make a “major announcement” on Saturday.

Daniel S. Goldman, a former federal prosecutor who helped lead the first House impeachment inquiry into former President Donald J. Trump, is also considering a run. Carlina Rivera, a city councilwoman from the East Village, filed paperwork on Friday with the Federal Election Commission to create a congressional campaign.

“This is a crisis moment in our nation for reproductive health, voting rights, L.G.B.T.Q. kids and their families, and climate change,” Mr. Hoylman, who would give up his Manhattan-based Senate seat to run, said in an interview. “I really do believe the moment demands the risk taking.”

There is also still an outside possibility that a sitting member of Congress could run for the seat to avoid a bitter primary. Mr. Nadler and Representative Carolyn Maloney have both declared their intentions to run in the 12th District for now, but parts of their old districts also comprise the new 10th.

Mr. de Blasio has made little secret that he wanted to continue to serve in elective office over the last year, but he passed over entering competitive Democratic primaries for governor and a congressional seat that would have joined Staten Island and Park Slope (the court invalidated the district).

He has tried to soften his image since leaving office, even acknowledging, in an essay in The Atlantic, his unpopularity with New Yorkers, suggesting that it stemmed from his failure to use “one of the loudest megaphones in the country.”

Mr. de Blasio won his first mayoral election with support from Black and liberal white voters. But he lost many of those white voters over the years, despite his success implementing universal prekindergarten, and Black voters make up just over 6 percent of the voting-age population in the new district.

“I just can’t identify who’s going to vote for him,” said Chris Coffey, the chief executive of Tusk Strategies, who lives in the new district.

One potential voter, Barat Ellman, a progressive rabbi who has lived in Park Slope since 1990, said she grew disillusioned with Mr. de Blasio’s approach to policing and criminal justice.

“There would have to be some pretty miserable alternatives for me to go with him,” she said.

The strength of Mr. de Blasio’s residual ties to the city’s ultra-Orthodox Jewish community, which played a key role in his rise, may also be in question.

Certain Orthodox leaders “may have benefited tremendously. But the average Moishe is very upset,” said Mr. Felder, a longtime critic of the former mayor who has not reached a final decision on his own campaign. “There’s a continuous issue of safety and security for the city. And if there’s no safety and security, there’s nothing else.”

In an interview, Mr. de Blasio acknowledged his political liabilities, which include tens of thousands of dollars in outstanding debts related to previous campaigns.

“Sure, but you also like the phenomenon of running in a race where a lot of people have voted for you before,” he said. “And it’s a new seat and a new role and I really believe voters look at each situation.”

On Monday, as the new district lines were being announced, he was eating a spinach ricotta crepe with a friend at one of his old college haunts — Caffè Reggio on Macdougal Street, within the confines of the new district. Within hours, his plans for a quiet few years of writing opinion pieces and appearing on television had changed.

“There’s a certain surrealism to this,” he said.