The Sixers billion-dollar plan for a new arena could come down to one person. And it’s not the next mayor.

City Councilmember Mark Squilla stepped onto the stage and faced the crowd at the Chinatown church hall — immediately pressed with questions about the Sixers’ intention to put a new arena on the southern edge of the neighborhood.

The surprise announcement that the Kenney administration wanted to quickly hire consultants to produce impact studies had angered Chinatown supporters, who said the $1.3 billion project was being rushed toward approval.

» READ MORE: Philadelphia officials say they will independently evaluate Sixers proposal for $1.3 billion arena in Center City

FACTS charter school director Ellen Somekawa told Squilla that a “slapdash, superficial, quick” vetting process served no one.

Squilla answered that it was the community’s desire for formal studies that drove the city’s request for proposals.

“We added the information that you wanted,” he told Somekawa.

“The thing is entirely not responsive to what this community was asking for,” she responded.

“I think it is,” Squilla told her.

The tension around that April 12 exchange at the Chinese Christian Church & Center came not just because Squilla represents the 1st District, where the arena would be located, but because of the ultimate importance of that reality. As the district Council member, Squilla holds the power to stop the arena in its tracks ― or set it on its way through the approval process.

» READ MORE: The Sixers want to leave South Philly behind. A Center City arena presents more questions than answers.

That may sound odd, particularly as the mayor’s race sharpens and candidates face questions on the Sixers’ proposal. But in Philadelphia, the tradition of “councilmanic prerogative” gives the 10 members who represent geographic areas the ultimate say on land-use decisions in their districts.

That means Squilla holds veto authority over the project and all it entails: the fate of a huge Center City investment, the future of Chinatown, the goals of a popular professional sports team, and the seemingly eternal, unrealized hopes for revival on East Market Street.

“I feel comfortable making that decision,” said Squilla, 60, “and it doesn’t weigh on me or lose any sleep over it.”

Councilmanic prerogative is a tradition, not a law or rule, that confers authority over property matters great and small, from the sale of public land to zoning changes to bike lanes.

All Council members generally vote with the colleague who holds the district seat, wanting the same courtesy for themselves, even when a project is of citywide importance. A 2015 report by Pew Charitable Trusts found that 726 of 730 of decisions were unanimous — 99.5% — and cited critics who said the prerogative too often serves the politically connected.

Squilla says it straight: He’s a proponent of prerogative.

“I know it’s a dirty word,” he said in an interview, but it makes Council members accountable to voters in their districts because those most impacted by a project “get a chance to vote for or against you.”

A spokesperson for the Sixers arena-development arm, in response to questions that included whether the team has considered ways forward if Squilla does not submit enabling legislation for the arena, said the partnership respects the Council member’s role.

So who is Mark Squilla, thrust to the center of a big and increasingly bitter debate?

People describe him as old-school South Philadelphia, proud of his heritage, born, raised, and living in the Whitman neighborhood. A job tending bar at Mick Daniels, at Second and Snyder Streets, helped him understand people and their problems.

Two years ago Squilla joined three Italian American groups in a losing lawsuit that claimed Mayor Jim Kenney discriminated by renaming the Columbus Day holiday as Indigenous Peoples Day. A few years earlier, he marked the summer opening of city pools by jumping into the water — clothed in dress shirt, tie, and suit pants.

People who know him say he’s a good and active listener. Returns phone calls and emails. Never one for the big floor speech.

He’s a moderate, to the point that some wondered if this year he might face a primary challenge from the left. (He did not.)

The Democratic majority whip is running unopposed for a fourth term in the May primary, where victory all but ensures election in November. He’s been mentioned as a possible successor to retiring Council President Darrell L. Clarke — and says he wants the job.

“Mark is accessible,” said Erme Maula, a South Philadelphia nurse and activist who works to improve access to parks and libraries. “I can literally text Mark right now, and he will text back in 10 minutes.”

Still, she said, that access means she can get broken lights fixed in a park, not that she or other grassroots advocates would be invited into inner-circle discussions.

Squilla began his life in politics as a Democratic committeeman, representing Ward 39B, a job centered on constituent services, which remains a primary focus.

Some Council members might wake up in the morning and delve into a wonky article in Governing magazine, said veteran City Hall lobbyist John Hawkins, head of Philadelphia Strategies Group. Squilla starts the day answering emails from constituents to help someone get an abandoned car towed off the street.

“Mark Squilla has always been concerned, first and foremost, about responding to his neighbors and community groups,” Hawkins said.

Squilla’s district stretches along the Delaware River from South Philadelphia through eastern Center City and Chinatown, north into Northern Liberties, Fishtown, Kensington, and Port Richmond.

In the last 20 years some of those neighborhoods have experienced enormous growth and change, and with that have come disputes around development, zoning, and quality of life.

Squilla, people say, favors compromise over confrontation, genuinely believing that reasonable give-and-take can produce agreement.

“He consults,” said Larry Freedman, the longtime zoning-committee chair of the Northern Liberties Neighbors Association. “Seeking consensus is not the easiest thing in the world.”

Freedman said he met Squilla when the then-new Council member committed a freshman mistake, making a promise he could not keep. Freedman can’t remember the details. The important part, he said, was Squilla came back to the community, admitted the error, and pledged to do better.

It showed a humanity and humility not always present in the attorneys and developers who sought the neighborhood’s approval, he said.

Squilla has been involved in some of the city’s most debated and controversial policies, introducing the $15 minimum wage bill for the mayor, supporting the soda tax, working to ban plastic bags. This year he found himself besieged by condominium and apartment-dwellers upset by a bill he introduced to require installation of sprinkler systems in older buildings.

Now he faces the challenge of his tenure, over the Sixers plan to build a privately funded arena four blocks from City Hall.

The Sixers intend to move in 2031, when their lease expires at the Wells Fargo Center in South Philadelphia. They say the new arena will bring investment, vitality, and foot traffic to East Market, and help move the region into the future with a reliance on public transportation.

The venue would stand on the footprint of 10th to 11th and Market to Filbert Streets, claiming the bus station and one-third of the Fashion District mall — and touching Chinatown at Cuthbert Street.

Chinatown surveys show 90-plus-percent disapproval, and two big protests are scheduled this week. Activists plan to deliver petitions to City Hall on Thursday and mount a march through Center City on Saturday.

Squilla has declined to state his opinion of the project, saying he’s continuing to gather information, hear community concerns, and await the outcome of formal impact studies.

Applause erupted at the Chinese Christian Church when speakers claimed he had promised to give Chinatown backers advance notice of government action — and that the search for consultants constituted action.

In fact, a review of recorded public meetings and remarks to reporters shows Squilla has consistently used the word legislation. That is, he said Chinatown organizations and supporters would receive any proposed arena legislation at least 30 days before it was submitted in Council.

Somekawa, who questioned him at the church, credits Squilla for traveling to Washington, D.C., to personally see how construction of a basketball arena harmed the Chinatown there.

“He expresses care and concern about Chinatown,” she said. “He’s made a number of commitments to the Chinatown community, which we’ve appreciated.”

At a Dec. 14 meeting at Ocean Harbor Restaurant in Chinatown, Squilla told the crowd that when he spoke to team development executives, he told them that in order for him to introduce legislation, they must meet with community members and work to understand and ameliorate neighborhood concerns.

“If they can, then a process would be in place to move forward,” Squilla told the crowd. “If they can’t, then the process would not move forward.”

Squilla also has said that a community benefits agreement must be signed for the project to advance. A CBA, as it’s known, is a negotiated contract in which a developer promises amenities or improvements, and in exchange, a neighborhood agrees to support the project.

The Sixers have offered a $50 million CBA that would go mostly to Chinatown. It’s unclear with whom the team might negotiate, given the views of residents and business owners.

In an interview, Squilla was asked if he saw room for common ground.

“As far as a compromise, I think anything could be worked out if there’s communication and people are talking,” he said. “There are challenges, and sometimes you could get to a point where you can move forward. But there are some times that you could get to a point where you can’t.”