SEATTLE (AP) — Dollar for dollar, the beleaguered movement to bring charter schools to Washington state has had no bigger champion than billionaire Bill Gates.

The Microsoft co-founder gave millions of dollars to see a charter school law approved despite multiple failed ballot referendums. And his private foundation not only helped create the Washington State Charter Schools Association, but has at times contributed what amounts to an entire year's worth of revenues for the 5-year-old charter advocacy group.

All told, the Bill and Melinda Gates Foundation has given about $25 million to the charter group that is credited with keeping the charter schools open after the state struck down the law, and then lobbying legislators to revive the privately run, publicly funded schools.

It's an extreme example of how billionaires are influencing state education policy by giving money to state-level charter support organizations to sustain, defend and expand the charter schools movement across the country.

Since 2006, philanthropists and their private foundations and charities have given almost half a billion dollars to those groups, according to an Associated Press analysis of tax filings and Foundation Center data. The review looked at 52 groups noted by a U.S. Department of Education website as official charter school resources in the 44 states plus Washington, D.C., that currently have a charter law, as well as the National Alliance for Public Charter Schools.

Most of the money has gone to the top 15 groups, which received $425 million from philanthropy. The Walton Family Foundation, run by the heirs to the Walmart fortune, is the largest donor to the state charter advocates, giving $144 million to 27 groups.

"We ought to be paying more attention to who these organizations are, and what kind of vision they have, and what drives them. A lot of these organizations have extraordinary influence, and it's often pretty quiet influence," said Jon Valant, an education policy expert at Brookings.

Charters aren't subject to the same rules or standards governing traditional public schools but are embraced by Gates and other philanthropists who see them as investments in developing better and different ways to educate those who struggle in traditional school systems, particularly children in poor, urban areas. Studies on academic success are mixed.

The charter support groups, as nonprofits, are typically forbidden from involvement in political campaigns, but the same wealthy donors who sustain them in many cases directly channel support to pro-charter candidates through related political action committees or their own contributions. In one indication of the philanthropy's success in asserting its priorities, Georgia's lieutenant governor was recorded saying he was motivated to support school choice laws to curry the Walton foundation's favor for his gubernatorial campaign. The Walton foundation has denied any connection to the candidate.

Nationwide, about 5 percent of students attend charters. They have become a polarizing political issue amid criticism from some, notably teachers unions, that they drain resources from cash-starved schools and erode the neighborhood schooling model that defines communities.

The Walton foundation notes the groups it funds have resources that often pale in comparison to the war chests of teachers unions, the usual foes in their battles over state education policy.

"The philanthropic support is essential for a small group of schools" that represents disadvantage families without their own political power, said Robin Lake, director of the Center on Reinventing Public Education, a University of Washington-affiliated think tank that has in the past been funded by the Gates foundation to support charter schools and traditional school districts working together.

But John Rogers, an education policy expert and UCLA professor, said it's a problem for democracy that billionaires who back a certain model of education reform can go toe-to-toe with a critical mass of professional teachers.

"A handful of billionaires who are advancing their vision of education reform is very different than having 200,000-some odd teachers across the state representing their understanding of public education through their union representation," Rogers said.

In California, the Waltons are the biggest backers of the powerhouse California Charter School Association, which has gotten more than $100 million since 2006 with support coming also from Gates, Michael and Susan Dell and the Mark Zuckerburg-backed Silicon Valley Community foundations.

"We're proud of our partners and very open about our desired outcomes, and that is, honestly, access to more better schools," said Marc Sternberg, who leads the Walton foundation's education program.

Sternberg said the foundation doesn't set the agenda but wants to empower the local vision, which has included the charter association's fight for more money and access to public school buildings through lawsuits against Los Angeles Unified, the country's second-largest school district. The California charter group said it works aggressively when painted into a corner.

A political arm of the association also has been a force in Golden State politics. It's now focusing on pushing pro-charter candidates in the November election, including former charter schools executive Marshall Tuck for state schools superintendent, and a number of legislative seats.

In Washington state, charter skeptics say Gates single-handedly propped up the entire charter school network. He gave at least $4 million to help pass a state charter school law, though the concept had failed three times at the ballot. Voters eventually approved a charter school law in 2012, making Washington one of the last states to adopt the schooling model.

After the state's highest court ruled in 2015 that the charter law's funding model was unconstitutional, the Gates-backed state charter group shepherded almost $5 million to keep the lights on at six charter schools and urged legislators to pass a new law. In 2016, its political arm called Washington Charters Action was created, and an affiliated political action committee has already given small amounts to dozens of state lawmakers up for election this fall.

Today, the state's teachers union is challenging the second version of the law. The Washington Educators Association's spokesman Rich Wood said the charter group inserted itself into the case after the union sued the state.

The Washington charter group — and all the charter schools in the state — wouldn't agree to be interviewed. The Gates foundation said in a statement it is not involved with the lawsuit but values the association's technical work helping charter schools blossom.

Some critics say money can define the advocacy itself, so not all charter support groups accept money from the billionaire philanthropists.

A second statewide charter support organization in California, the Charter Schools Development Center, relies on programming fees to preserve its independence, according to director Eric Premack.

Though the two California charter groups share many similar values, Premack said, they're on different sides of the testing issue: how to and how much to use test scores to determine educational quality. Premack said he rejects test-based accountability — embraced by the California Charter Schools Association and many of its business mogul donors — as antithetical to the charter movement's innovative spirit.

"You often find them being close political bedfellows — if not the same — who support high-stakes testing," Premack said.

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Associated Press journalist Larry Fenn in New York contributed to this report.

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Follow AP Education Reporter Sally Ho on Twitter: https://twitter.com/_SallyHo