Mike Bloomberg, the billionaire former mayor of New York City who briefly ran for the 2020 Democratic presidential nomination, announced Wednesday in the Wall Street Journal that he was donating $750 million to help grow charter schools in 20 metropolitan areas around the country.

Declaring that “American public education is broken,” Bloomberg wrote in a Journal op-ed that he was donating the money over five years “to support the success and growth of existing charter and autonomous schools, open new high-quality charter schools, and create city- and state-level conditions that will help sustain this progress.”

The plan, according to the Bloomberg Philanthropies website, is designed to create 150,000 new seats for students in charter schools, which are publicly funded but privately operated. New York City is one of the cities that will benefit from his donation, but the others have yet to be determined.

The size of the donation is significant. By way of comparison, Congress allocated $440 million for the U.S. Charter School Program in 2021, which supports grants to improve existing charters and open new schools, and President Biden proposed the same amount for 2022. If that amount stayed constant for five years — the period for which Bloomberg’s donation covers — it would amount to $2.2 billion. Bloomberg’s $750 million would be about a third of that, an enormous investment for a private individual.

Bloomberg’s newest philanthropy extends his longtime interest in promoting charter schools and underscores how the mega-wealthy in this country have for decades been using their private fortunes to influence public education.

The Bill & Melinda Gates Foundation has been by far the biggest education philanthropy over the past few decades, spending billions on various projects to change schooling. Its efforts have fueled a national debate about wealthy private individuals’ impact on public policy.

Bloomberg’s investment comes at a time when the charter school movement has come under scrutiny from public education advocates, who say that some of them take needed public funding from traditional districts that educate most children. Biden said last year during the Democratic primaries that charter schools “siphon off money for our public schools,” and he vowed to end federal funding for charter schools operating for profit.

Critics have also expressed concern that many charters operate with little or no oversight from governing bodies — resulting in financial scandals — and are not transparent about how they use public funds.

Charter advocates say these schools offer choices to families who want alternatives to troubled schools in traditional districts that do not provide adequate academics and other services. They also say charters perform better than traditional schools. Bloomberg called charters a “proven alternative” to traditional public school districts.

That ignores research that shows that overall, charter students do not perform any better on standardized tests than students in traditional public schools. Like traditional public schools, charter schools range in quality.

Before the pandemic, about 7 percent of U.S. schoolchildren attended charter schools, with 44 states plus the District of Columbia, Guam and Puerto Rico having laws that permit them. A recent report by a charter advocacy group showed that charter school enrollment had jumped 7 percent during the pandemic as some charters were able to provide better services for students than neighborhood schools did. (Some of that increase was in online charters that charter school supporters themselves say are not adequate.)

Nina Rees, chief executive of the National Alliance for Public Charter Schools, an advocacy group, praised Bloomberg, saying in a statement that his investment “signals that government, politics, philanthropy, and the nation are taking notice of how charter schools can be a lifeline for so many students.”

Carol Burris, executive director of the Network for Public Education, an advocacy group that opposes the spread of charter schools, said in a statement that she disagrees with Bloomberg’s statement that public education is broken and is “shocked that he would use the tragedy of covid to further what has been his long-held agenda, which is to replace public schools with charter schools.”

For years, Bloomberg has funded local school board candidates across the country who support charter schools. In 2013, for example, he donated $1 million to a pro-charter candidate running for the Los Angeles Board of Education, who ultimately lost.

Bloomberg was mayor of New York City from 2002 to 2013, a period in which he led school reform in the country’s largest district that included the expansion of charter schools. He waged a campaign for the Democratic presidential nomination from November 2019 to March 2020.

The Forbes list of billionaires ranks Bloomberg as the 20th-richest person in the world, with $59 billion, though he is not listed on Bloomberg.com’s own list of the wealthiest people.

As large as Bloomberg’s pledged charter investment is, it is not his biggest in education. He gave $1.8 billion to Johns Hopkins University in Baltimore for student financial aid at a time when he was considering running for the Democratic presidential nomination.

(Update: Adding Burris comment)