The city’s charter school sector is pushing back against a groundswell of support for a moratorium on the space-sharing arrangement that has allowed the schools to proliferate.

Their resistance is not unified in tone. Some charter school advocates are requesting that proponents of a moratorium reconsider and others are taking their fight to the street.

The Bloomberg administration has relied heavily on co-location, the practice of allowing one school to open in another school’s building, to open new schools. Its critics say the arrangement breeds unnecessary tension and takes resources away from existing schools.

Now, three of the Democratic candidates for mayor — Public Advocate Bill de Blasio, Comptroller John Liu, and former comptroller Bill Thompson — have all said they think Bloomberg should not be allowed to close or co-locate any schools in its last year. A bill proposed in the State Senate would bar school closures even into the next mayor’s term, and Assembly members are lining up to sponsor their own version of the bill.

Blocking co-locations and the school closures that often make space for them would be a serious blow to the city’s charter sector, which has flourished because the Bloomberg administration has offered more than 100 charter schools free space in district buildings. It would be difficult for new schools to open at the same pace if they had to find and pay for private space.

The threat has united independent charter schools and schools in management organizations, which are sometimes at odds, in the sector’s defense. On Wednesday, two dozen school leaders and advocates distributed a statement asking the mayoral candidates “to set aside the call for a moratorium on co-locations and show the kind of thoughtful leadership New York City needs.”

Several of the signatories handed out flyers to people attending the Council of School Supervisors and Administrators’ event featuring mayoral candidates on Wednesday evening, where the candidates later reiterated their support for a moratorium.

“I believe anyone who wants to lead the city is going to be a thoughtful person,” said Rafiq Kalam Id-Dinn, head of Brooklyn’s Teaching Firms of America Professional Preparatory Charter School. He said that if candidates surveyed city charter schools’ strengths, they should think, “Hmm, maybe we shouldn’t push pause.”

Some charter school operators are taking a more aggressive stance. Unlike representatives of the other major charter school networks in the city, Success Academy Charter Schools CEO Eva Moskowitz did not sign on to the sector’s letter. Instead, parents from her schools protested outside the three mayoral candidates’ offices this morning.

Outside Liu’s office, more than 100 parents from Success waved signs accusing the candidates of being pawns of UFT President Michael Mulgrew. The union has long opposed closures and co-locations, at times suing to stop the city from going through with them.

“We’re out here because we want co-location,” said Ali Aybakal, who has a child at Harlem Success Academy 4 in Harlem and three at New Heights Academy.

“Liu, Thompson, and De Blasio declared basically to destroy charter schools,” said Kelly Alday, an outspoken parent from Bronx Success Academy who is often a ringleader at the network’s protests. “They’re basically turning their backs on us.”

Moskowitz, who aims to open six new schools in public space this fall, has long represented the more radical wing of the charter movement, bringing busloads of parents to defend her network’s schools at public hearings and meetings where criticism is likely.  A former City Council education committee chair, she has also sought — and received — more space from the city than any other charter school operator, at times forcefully proposing space-sharing plans directly to the chancellor.

Now, she is taking an extra share of criticism about the practice of co-location.

“Another thing that has to change starting in January is that Eva Moskowitz cannot continue to have the run of the place,” de Blasio said during the mayoral debate, where Moskowitz was the only school leader named, to loud applause. “She was giving the orders and chancellors were bowing down and agreeing. That’s not acceptable.”

In a statement responding to this morning’s protests, Melinda Martinez, a parent at Cobble Hill School for International Studies, said, “The Bloomberg-era policy of ‘what Eva wants, Eva gets’ is at stake now, and only the mayoral election can stop her from continuing to hurt our students, because not everyone can be bought off.” Success Academy Cobble Hill moved into the school’s building this year.

Perhaps anticipating this year’s political rancor, many independent charter schools opted out of a rally to support the sector last year. “There was a sense that there is a political element to this, and people thought that demonstrations that looked like Eva’s demonstrations did more to divide than bridge,” Harvey Newman, who heads the Center for Educational Innovation’s charter support network, told GothamSchools at the time.

Charter school parents – who number over 100,000 and counting — could potentially be a significant voting bloc in the mayoral election. At an event to mark National School Choice Week this morning at Democracy Prep Charter School in Harlem, a parent asked Chancellor Dennis Walcott what families can do to support the sector. Walcott suggested holding a forum for mayoral candidates and giving them each a report card based on how much they favor charter schools.