At last year's rally, students and families protested in Harlem against a lawsuit that sought to prevent charter school co-locations.

A large public rally to support the city’s charter school sector this afternoon is expected to draw thousands of people, but the event is also notable for who won’t be there.

Organizers say the rally is meant as a show of political might to mayoral candidates, whose support for the sector is unlikely to rival Mayor Bloomberg’s.

But in a sign of what sources say is a widening rift within the sector, two major groups that support charter schools have declined to participate, and large numbers of independent charter school operators are sitting the rally out. Many say they believe the event’s leadership and timing reflect a larger truth about the future of the sector: that it is promising for schools that are part of large networks and less so for independent charter schools.

“The charter world is kind of breaking up into the haves and the have-nots. There’s a schism,” said a source with a long history in city charter schools.On one side of the schism are operators, many of them independent, who have focused their energies and resources on school-based operations while quietly steering clear of front-line battles over ideology. On the other side are operators who also see charter schools as a weapon in a political fight against teachers unions to reform the larger school system and believe that the fight requires robust, hands-on organizing and lobbying efforts.

In recent years, operators who hold that view have seen their networks grow and win support from well-funded advocacy groups, particularly as many of their schools have outperformed their local districts and the city on state tests.

Several of the advocacy groups spearheaded today’s rally, set for City Hall Park at 3 p.m. Education Reform Now, which former Chancellor Joel Klein chaired until April, secured a permit for a crowd of 3,000 and is supplying the event with volunteers. Families for Excellent Schools, a parent organizing group, arranged busing and coordinated a steering committee. The five-member committee included representatives of four high-profile networks — Success Academy, KIPP, Uncommon Schools and Public Prep — and one independent school, Coney Island Preparatory Academy.

Last year, the charter sector held a 2,500-person-strong rally against the UFT and NAACP, which had filed suit to stop 18 charter schools from opening or expanding in city school buildings. Without the common foe, and with the prospect of competition for scarcer resources on the horizon, that unity has frayed this year.

“We have … chosen not to participate in this or any other event that perpetuates the exclusion of independent charter schools from leadership in directing and planning these events,” said Rafiq Kalad Id-Din, founder and principal of Teaching Firms of America Charter School, who helped organize last year’s rally.

Critics of today’s event said its leadership had created a major wedge.

Harvey Newman, who heads the Center for Educational Innovation’s charter support network, said the school leaders in his coalition felt the event was too closely aligned to the political agenda of Eva Moskowitz, the founder and CEO of the Success network. A former City Council education committee chair, Moskowitz 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.

Newman said a representative of Moskowitz’s network approached him about signing on to the rally just days after Moskowitz indicated that she was considering a run for mayor.

“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,” said Newman.

Organizers for today’s event downplayed the tension and said the dissent did not represent a majority view.

“This is NOT about Eva in any way and it’s unfortunate that a tiny number of people feel that way,” Barbara Martinez, a spokeswoman for Uncommon Schools, wrote in an email. Eighty of the city’s 136 charter schools are expected to be represented at the rally, she said.

“It’s to show folks who might be running for mayor that there is a large number of us who support charter schools,” said Martinez. “We want to show them that we believe that some of the policies in place, such as co-location, works.”

But co-location — the city’s practice of handing space in public school buildings to charter schools at no cost — is precisely one of the issues where independent charter school operators say they have been pushed to the side.

A larger proportion of independent charter schools are housed in private space, and an operator who runs a school that did not get public space said high-profile charter networks were unfairly given “the first pick of the litter” when the city allocates space. Last fall that dynamic played out in District 15, where a long-planned independent charter school seeking public space was denied to make way for a Success Academy school.

Preserving co-location, which allowed charter schools to thrive under the Bloomberg administration, is the sector’s preeminent concern for the next mayoral administration.

“That’s definitely a key element that I think the group is going to want to convey,” said Coney Island Prep founder Jacob Mnookin about today’s rally, which he helped organize.

But even the leading charter school advocacy group in the city, the New York City Charter School Center, is staying away from the rally, sources close to the group confirmed.

CEO James Merriman said he was “supportive of schools and parents coming together to publicly support charter schools” but declined to comment on today’s event. Multiple sources said the center, which has lent resources and support to rallies in the past, had decided against participating in today’s event because of the leadership and timing tensions.

Several prominent charter network operators who serve on the center’s board are absent from the rally’s list of participants as well, including Jeff Litt of Icahn Charter Schools, Geoffrey Canada of Promise Academy Charter Schools, and Joseph Reich of Beginning with Children Charter Schools.

Also not on the agenda is Newark Mayor and “ed reform rock star” Cory Booker, who was billed as the keynote speaker in an email announcement sent late last week. The New York State Charter Schools Association sent the alert prematurely and had never confirmed Booker’s attendance, according to an official there.

Booker’s possible appearance triggered charter school opponents to organize a counter protest at the same time and place this afternoon. After learning that Booker would not attend, they sent out an updated media alert: “Keynote Speaker Cory Booker a No-Show; Why is He Hiding?”

Mnookin, of Coney Island Prep, said it’s essential that tensions within the charter sector not overwhelm the schools’ shared interests.

“Obviously, we have incredibly diverse and varied schools,” Mnookin said. “But the opportunity to come together as a united group is a huge benefit.”

The event falls at a time when most charter school days aren’t over and with the school year winding down, organizers said they didn’t expect every charter school to be represented at the rally.

Some charter school parents and organizers said they had gotten very little information about the rally.

A parent at a school that is not part of a network said parents had received a generic invitation through email late this morning. The invitation, a Word document created by Families for Excellent Schools, included the words “Insert school logo” and billed the rally as an end-of-year celebration.

“I don’t even know who is sponsoring it or what it is about, just that we are invited,” said the parent, whose school appeared on a list of confirmed participants provided by a rally organizer. “I doubt anyone would show with such little info.”

And Joanne Hunt, the principal of the independent Harbor Science and Arts Charter School, one of the city’s longest-running charter schools, wrote in an email that she hadn’t heard about the rally. “Was it publicized?” she asked.