BOSTON (AP) — Legislation unveiled Thursday would let the state add up to a dozen new or expanded charter schools each year outside the existing cap on the schools in Massachusetts.

Gov. Charlie Baker said his bill would focus the new growth in districts that are performing in the bottom 25 percent of districts statewide.

The bill would also let the schools give preference to high-need and low-income students, as well as students learning the English Language and those with learning disabilities.

Baker, a long-time supporter of charter schools, said the legislation will help give every child the opportunity to a high-quality education regardless of their location or background. The new or expanded schools he is proposing would not count against the current charter school cap of 120 schools.

About 80 charter schools currently operate in Massachusetts.

Opponents said the proposal — which echoes the language of a proposed 2016 ballot question — in theory could allow the state to add new charter schools each year until no district public schools remain.

Massachusetts Teachers Association President Barbara Madeloni said Baker's bill would help create a two-tiered education system, one truly public and the other private, but financed with public dollars.

"The truly public system will always welcome all students. The private system will continue to find ways to underserve those with the most needs and then use inflated claims of success to grab an ever-larger share of public education funding," Madeloni said in a written statement.

Baker made the announcement Thursday at the Brooke Charter School in Boston.

"People have asked me many times, what's the right number of charter schools? I don't know the answer to that, but we have 37,000 families who simply want for their kids what everybody else wants and for the most part gets here in the Commonwealth of Massachusetts for theirs — which is a shot at a great education and an opportunity on which to build a great future," Baker said.

More than 37,000 students were placed on waiting lists for charter schools for the 2015-16 school year.

The possibility of a 2016 ballot question that would also allow the state to add 12 new charters schools outside the existing cap could help strengthen Baker's hand in any negotiations with Democratic leaders in the House and Senate on his bill.

There is precedent for using impending ballot questions as leverage. Last year, for example, a group that supported a higher state minimum wage ended its bid to put a ballot question before voters, saying it was no longer necessary after lawmakers approved a bill intended to give Massachusetts the highest minimum wage among states by 2017.

There is also a pending lawsuit by lawyers representing five Boston students aimed at lifting the charter school cap.

Baker said his bill would let charter schools use a weighted lottery system that provides additional weight to high-need and low-income students, as well as students within a particular geographic zone.

He said the bill would also let charter schools enter into voluntary agreements with districts to participate in district student enrollment systems that take into account parental preference. The legislation would also create new opportunities for charters to partner with districts in turning around their lowest-performing schools.

Baker said that part of the bill would let the charter school operators take some of the tools that they developed to help turn around those lower performing schools.

The first charter schools opened in Massachusetts in 1995, two years after the 1993 law that allowed their creation.

The majority of the schools in Massachusetts are known as Commonwealth Charter schools, operated independently of local school districts and union contracts. The state also has a handful of Horace Mann Charter Schools that operate with approval of the local districts and their teachers' unions.