Massachusetts has become a major battleground for corporate education reformers intent on expanding the charter sector this fall. Their goal: to lift the 120-school statewide cap on charter schools. Their vehicle: a binding referendum on the ballot on Nov. 8.

Massachusetts public schools are ranked No. 1 in the nation by many measures, but charter school advocates, led by the hedge fund-backed Families for Excellent Schools, are flooding the airwaves with TV commercials calling for the state to be allowed to authorize an additional 12 charters each year.

“My blood pressure just went up because I saw another commercial,” said Richard Stutman, the president of the Boston Teachers Association. “Families for Excellent Schools publicly said they were investing $18 million, but they’ll be well beyond that when it’s over.”

Families for Excellent Schools, which is based in New York City, spends millions of dollars each year in New York seeking to sour public opinion on public schools as it vigorously lobbies Albany to expand the number of, and funding for, charter schools. This year, it set the goal of doubling the number of New York City students in charter schools to 200,000.

The organization’s latest campaign in Massachusetts is indicative of its larger ambitions of unfettered charter growth nationwide.

As a nonprofit with 501(c)(4) status, Families for Excellent Schools does not have to reveal donors, creating what many people call “dark money” — funds contributed by individuals and companies with agendas that are safely hidden from the public eye. Other big donors to the campaign to lift the cap in Massachusetts include Alice and Jim Walton of the Walmart fortune (donations of nearly $2 million combined) and former New York City Mayor Michael Bloomberg, a Boston native, who gave $240,000.

With the backing of current Republican Gov. Charlie Baker, charter advocates first sought and failed to prod the Massachusetts Legislature to lift the charter cap in 2014. Pro-charter groups also filed a lawsuit in 2015 in state court challenging the constitutionality of the cap. The case was thrown out this October by a judge who ruled that the state had a “rational basis” for limiting the size of the charter sector.

Charter advocates now see this November’s referendum, in which they are using their deep pockets to try to influence voters, as their best chance to get the cap lifted. But the opposition, led by the Massachusetts Teachers Association and the Massachusetts Federation of Teachers, has held its own despite being greatly outspent. Most polls show growing opposition to the ballot initiative.

“Our message of public schools losing funds to charter schools is a powerful message,” said Stutman of the Boston Teachers Association.

Save Our Public Schools, the coalition of labor and community groups leading the “no” campaign, has calculated that if the charter cap is lifted, 231 local school districts will lose $450 million in funding to charter schools in 2017.

Barbara Madeloni, the president of the Massachusetts Teachers Association, said the unions and their allies have focused on grassroots campaigning.

“Our strategy was to put our energy into groundwork, phone banks and canvassing,” Madeloni said. “Our members, teachers who have been so beleaguered, get to go out and talk to people and establish communication with the community around public education.”

The amount of outside money flooding into the “yes” campaign also has raised the hackles of Massachusetts voters.

“I’m not unhappy Families for Excellent Schools is from New York,” Stutman said. “The outside influence has everyone’s attention.”

Kelly Henderson, a high school English teacher for 11 years in Newton, Massachusetts, said it’s plain to teachers what is going on.

“I’ve seen the hands of corporate America creeping into our schools,” she said. “This is the culmination of the efforts of those forces to take over. This is a threat to public education as a public institution.”