If you heard that a group of philanthropists came together to donate millions of dollars to schools, you would probably consider it good news. Indeed, thousands of underprivileged kids will be helped by the $35 million raised for Success Academy charter schools at a charity gala earlier this month. But teachers unions detect a nefarious purpose. This $35 million donation was “part of a coordinated national effort to decimate public schooling,” Randi Weingarten, president of the American Federation of Teachers, wrote in an April 13 article at the Huffington Post. “Wealthy donors and their political allies,” she warned, are “pushing unaccountable charter growth in urban centers while stripping communities of a voice in their children’s education.” Regardless of the political attacks, politicians and philanthropists must remain committed. Charter schools serve many underprivileged students: 56% are on free or reduced lunch and 65% are minorities, according to the National Center for Education Statistics. Because they are run independently of school districts and city bureaucracies, they have the flexibility to be innovative in the choices they offer to parents, providing services like extended-learning schedules and language immersion. Charter schools are also closing achievement gaps. At Success Academy schools in New York, three-quarters of students qualify for free or reduced-price lunch and nearly all are minorities. In 2015, 68% of students scored proficient in reading and 93% ranked proficient in math. For contrast, only 35% of New York City students overall scored proficient in math. Their reading abilities were even worse. This success translates to broad-based support. About two-thirds of public-school parents favor charter schools, according to a 2015 Phi Delta Kappa/Gallup poll. Support is especially high among low-income parents, according to a March survey commissioned by the organization I lead. Some 88% of parents who earn less than $50,000 a year would like to see more charter schools in their communities. Union leaders haven’t always been adamantly anti-charter. Ms. Weingarten’s former boss and mentor Al Shanker is actually credited with proposing charter schools. Sharing his vision in a 1988 speech, he said, “There is a role in all this for the federal government, state government, the local government, the business community, and foundations.” Today, 25 years after Minnesota passed the first charter-school law, nearly three million students attend about 7,000 charter schools in 43 states and the District of Columbia. Yet over one million students remain on charter waiting lists, meaning that additional schools can’t come soon enough. And because charters nationwide receive, on average, 72 cents for every dollar that district-run schools do, philanthropy is vital to expansion. Philanthropists have always contributed to their alma maters and other civic institutions, but opportunities to support public education have been limited. Donors want their contributions to have measurable results, and few successful businesspeople would voluntarily send money to poorly performing district bureaucracies. Mark Zuckerberg learned this lesson the hard way when much of his $100 million gift to public schools in Newark, N.J., was frittered away. Charter schools have changed the equation for wealthy donors aiming to improve education. In Los Angeles billionaire Eli Broad is backing an effort to raise $490 million to create 260 new charter schools for more than 130,000 students. Yet entrenched interests seem more concerned about explaining away the failures of public schools than supporting innovative ways to help students learn. In Louisiana, Gov. John Bel Edwards threatened to severely limit the ability of the state board of education to authorize charter schools rejected by local school boards. This appeal role of the state board is important because it ensures that quality charter schools can open even if local politics prevent approval. Gov. Edwards’s proposal was defeated in the state legislature, but the episode demonstrated how a single official could jeopardize years of progress. New Orleans’s all-charter district has been catching up with the rest of the state and has raised graduation rates by 10 percentage points over the past decade. In Massachusetts, Gov. Charlie Baker is fighting to lift an arbitrary cap that limits the state to 72 charter schools. The Massachusetts Teachers Association is spending millions to keep the caps in place. This despite Boston charter-school students gaining 170 days of extra learning in reading and 233 days in math, compared with regular students, according to a report by Stanford University’s Center for Research on Education Outcomes. Charter schools put high-quality education within reach of students without regard for family incomes. Policy makers and philanthropists should pay close attention to how these schools are revamping communities and attracting philanthropic investment to some of the neediest neighborhoods. Charters have the potential to revolutionize American education—but they will need support to do so.