In 2013, Bill de Blasio campaigned for mayor on a promise of universal pre-K. Two years later, New York City enrolls more children in full-day pre-K than any state except Georgia, and its preschool enrollment exceeds the total number of students in San Francisco or Boston.

But quality costs money — $9,076 per student per year, according to a report by two groups, The Institute for Women’s Policy Research and Early Childhood Policy Research.

Few states are willing to make that kind of commitment. Florida, the only state to deliver preschool on a scale and at a speed comparable to New York City, offers a cautionary lesson.

In 2005, voters there made universal prekindergarten a constitutional right. But quality suffered because the state spent a meager $2,238 for each 4-year-old in 2013-14, largely by using underpaid and poorly trained teachers.