On Thursday Mr. de Blasio went on a sympathetic radio station and couldn’t have been clearer about what is driving his actions. Charter schools may help the poor and those just starting out in America, they may give options to kids who’ve floundered elsewhere, but a lot of them are supported by rich people. There is a “strong private-sector element” in their funding, he said. The mayor agreed with host Ebro Darden that “a lot” of charter schools are funded by big business: “Oh yeah, a lot of them are funded by very wealthy Wall Street folks and others.” When Mr. Darden and co-host Peter Rosenberg suggested that a “campaign” to portray the mayor as anti-charter-school was also funded by big business, Mr. de Blasio, as the New York Post noted, didn’t disagree. “I think you’re providing a keen political analysis there.”…

It is true that wealthy and public-spirited New Yorkers, out of loyalty to the city and its future generations, give a lot of money, care and time—the last, time, doesn’t get noted enough—to create and help run many of the city’s charter schools. They should be thanked for this, every day. Again, they do it because they care about children who would otherwise be locked into a public-school system that doesn’t work.

But the people who run the public-school system that doesn’t work—the one where you can’t fire teachers who sexually prey on students and principals who don’t even show up for work, which is to say the public schools run by the city’s huge and powerful teachers union—don’t like the charter schools. And they are the mayor’s supporters, a significant part of his base.

The very existence of charter schools is an implicit rebuke to the public schools. It means they are not succeeding, and something new must be tried. That something new won’t be perfect—no charter school is, and some are more imperfect than others—but people still line up to get into them. And there’s something to the wisdom of crowds. When a school exists for the students, you can tell. When it exists for the unions, you can tell that too.