Charter schools are a success story in New York City, a town that's desperate for educational progress. So why does Bill de Blasio want to make life harder for them?

The Democratic mayoral candidate has said he doesn't think the city needs more charter schools, and he wants to charge charter operators rent for the space they share with district schools in city-owned buildings.

We can't think of a better way to kill innovation or roll back progress in the public schools. The city has 183 charter schools serving 70,000 students among a total public school enrollment of 1.1 million. It's not like the charters have morphed into a resource-eating monster.

And in fact, the demand for charter slots is building -- with 53,000 kids now on waiting lists. Parents know where the educational progress is happening.

While some charter schools do better than others, the movement is showing undeniable results.

In neighborhoods where the most charters are situated -- the South Bronx, Harlem, central Brooklyn -- charter schools chalked up higher scores in last spring's statewide proficiency exams than their district counterparts.

As New York State Board of Regents Chancellor Merryl Tisch points out, charter schools are proving their worth as crucial alternatives to failing schools.

Joe Lhota, the Republican mayoral candidate, has said he would double the number of charter schools and continue to let them use city school buildings rent-free.

While the parents of district-school students worry that charters are taking up valuable classroom space in their buildings, it's worth noting that the charters are public schools, too, teaching local children in community spaces.

De Blasio would have the charters pay rent on a sliding scale. The charter networks with the strongest private contributor bases would pay the most.

We only see one real reason to do this -- and that's to make the United Federation of Teachers happy.

The union has never liked the idea of charters, whose teachers do not necessarily have to abide by UFT rules. The more that charters succeed -- and they're a major hit -- the more the UFT seems to worry.