The battle has begun. Schools Chancellor Carmen FariƱa has said the city will divert $210 million intended for charter schools to pay for prekindergarten seats.

It seems City Hall is determined to do a good thing as badly as possible. The citywide expansion of pre-K is a great idea. But why must it come at the expense of another crucial idea -- charter schools? It's not as if urban systems have an overabundance of promising strategies to nurture and maintain student performance.

Mayor Bill de Blasio explains that city schools have an overcrowding problem today -- and that's before the pre-K buildout even starts. The $210 million siphoned away from the charter program can help, he believes.

We have a better idea. Why not let charter schools play a stronger role in the city's pre-K expansion? Why not let them offer their own pre-K classes? The charters have done some of their best work in low-income minority neighborhoods -- where pre-K classes are needed most.

A natural fit, no?

No. It's a union thing. The United Federation of Teachers is unhappy because instructors in many city charters -- which are public schools -- are exempted from UFT work rules. The UFT is also upset because charters share space in public school buildings without paying rent. The union wants that changed, and de Blasio hasn't been shy about agreeing with them.

If you listen to the mayor talk, you'd think the charter supporters are in the same category as the much-maligned 1 percent -- the plutocrats de Blasio wants to tap to pay for pre-K. But they're not. They're the mothers and fathers in working-class neighborhoods throughout the city who want to give their children the best education they can get. More than 50,000 city kids are on charter waiting lists.

The public discussion of this issue has been shamefully unhelpful. Charters have made crucial contributions to education reform. So has pre-K. The unions are fighting changes they find threatening, and de Blasio has mistakenly been their ally. That's a lousy way to seek reform.