With the press on from charter school advocates to lift or eliminate the cap on charter enrollment in poorly performing school districts, the battle brewing over the issue in the Legislature appears to have as much, if not more, to do with the distribution of state education aid than the merits of charter schools.

The Race to the Top Coalition staged a rally at the State House on Wednesday, urging a House-Senate committee to recommend a bill filed by Rep. Russell Holmes, of Boston, and Sen. Barry Finegold, of Andover, to eliminate the cap in underperforming school districts and give schools on the cusp of being labeled underperforming more autonomy over hiring, curriculum and other functions.

Though a bill could surface anytime between now and the end of session in July, the Education Committee faces a March 19 deadline to report on bills pending before the committee. Holmes and Rep. Paul Schmid (D-Westport) were the only two lawmakers to attend a portion of the rally.

“Why put a cap on success?” Boston Foundation President Paul Grogan asked, citing a study done by Stanford University that found students in Massachusetts charter schools gained one-and-half months of learning in reading and two-and-half months in math by attending a charter for one year.

“This is not an assault on the regular Boston or Massachusetts public schools. In fact, it is just the reverse. What we’re finding over and over again is that the existence of a thriving charter school movement has helped improve the regular district schools dramatically in Boston as the lessons that the charters offer are being adopted at a swift pace,” Grogan said.

But much of the opposition has less to do with the successes of some charter schools than the state’s underfunding of its charter school reimbursement program. That program is intended to refund part of the local aid lost by public school districts to charter schools.

“The first priority needs to be having the Legislature and the governor fully fund the charter school reimbursement program. If they fully fund the statutory formula that’s designed to reimburse just a portion of the Chapter 70 money we wouldn’t have objection at all to raising the cap. The problem now is cities and towns are being fiscally harmed by the loss of Chapter 70 money to charter schools,” said Geoff Beckwith, executive director of the Massachusetts Municipal Foundation.

Charter schools are paid by the district from which they take students 78 percent of the average per-pupil cost, and the state is supposed to reimburse school districts 100 percent in the first year and 25 percent each of the following five years. The program, however, has been underfunded in fiscal 2014 by $27.8 million, with $75 million appropriated for reimbursements that would total $102.8 million if fully funded.

Beckwith said that even though charter schools are taking on the responsibility and per-pupil cost of educating students, public school districts are not able to simply cut teachers or fixed costs like electricity when students leave for charter schools. The lack of full reimbursement, according to the MMA, has cost Boston $10.3 million in state aid. In Lowell, the lost aid amounts to $1.4 million and in Springfield it totals $1.5 million.

“There are major fixed costs that cannot be reduced to just offset the loss of the Chapter 70 money,” Beckwith said. “Charter school expansion will continue to put financial pressure on cities and towns, but that pressure increases exponentially if the reimbursement formula is not fully funded.”

Sam Acevedo, executive director of Boston Higher Education Resource Center, said eliminating the cap in underperforming districts will “work wonders” for students.

“There is no moral reason for our legislators not to enact this legislation,” Acevedo said.

The rally also featured leaders of charter schools from around the state who attested to the success of their programs, and pushed back against the notion that charters don’t accept the types of students with disabilities or language barriers that can create challenges at traditional schools.

Sonia Pope, executive director of Holyoke Community Charter School, said 88 percent of her charter school, which serves students from kindergarten through eighth grade, are Latino and 85 percent come from low-income families. Holyoke Community Charter, with its 702 students, wants to expand with a high school, but the city is at the cap for charter school seats and Pope’s school needs at least 200 additional seats to get a high school off the ground.

“We want to assist the public school system, which is underperforming,” Pope said. Eighth grade students at Holyoke Community Charter perform 21 percent better on the English language MCAS exam and 34 percent better on the math exam than the rest of the state, according to Pope.

Shannah Varón, executive director of Boston Collegiate Charter School, said her school’s lottery, like charters across Boston, will take place on Wednesday afternoon. Varón said 900 names will go into a box and only 50 slots will be offered to students whose names are drawn.

“It’s an exciting day, but it’s a bittersweet day for us,” Varón said.

Students at Boston Collegiate Charter spend, on average, 30 percent more time in the classroom than those in Boston public schools and participate in afternoon tutoring sessions and other programs.

“We barely let them go home,” Varón said, saying her school is not a threat to traditional public school or a substitute for high performing non-charter public schools.

Though charters still lag behind traditional public schools in the numbers of students enrolled with learning disabilities or language skill gaps, supporters argued that great progress has been made since 2010.

“That’s like saying I used to have hair and now I’m losing it,” said Richard Stutman, president of the Boston Teachers Union.

Accusing charter schools of having “discriminatory enrollment practices,” Stutman said charter schools open their doors to all students but “miraculously” a large number of those with disabilities or those who speak English as a second language end up dropping out.

“If the charters accepted and retained student populations that were consistent with district wide percentages, I think people would have a better view of them. They just don’t do it,” Stutman said.

Paul Toner, president of the Massachusetts Teachers Association, said he’s worried the bill would further the creation of a “parallel school system” rather than encourage school districts to more quickly adopt charter school strategies that are working.

Monalisa Smith, the president and CEO of Mothers for Justice and Equality, said parents just want the opportunity to make a choice for their children where they attend school. “If public schools want to retain our children, then they need to raise the bar,” she said.

Felix Browne, vice president of policy and communications at the Massachusetts High Technology Council, said charter schools have been successful preparing students to compete in the global marketplace.

“The legislation currently before the Education Committee represents an opportunity to unlock the potential of many more promising students and ensure that our pipeline of talent that we keep them here and that we have well prepared students and that there able to work in these high quality high paying jobs here in the Commonwealth,” Browne said.