For several months running, the Bill and Eva Show has been the talk of New York City politics. He is the new mayor, Bill de Blasio, an unapologetic old-school liberal Democrat, scourge of the rich and of public charter schools. She is Eva Moskowitz, fellow Democrat and educational-reform champion who runs the city's largest charter network.

How did Ms. Moskowitz, a hero to thousands of New Yorkers of modest means whose children have been able to get a better education than their local public schools offered, end up becoming...