When Sen. Elizabeth Warren released her education plan, she trotted out a familiar charge against charter schools: that they “strain the resources of school districts.” To fight this supposed scourge, she promised to end federal financial support for new charter schools. And she’s not an outlier among the Democratic presidential hopefuls. Her fellow progressive Sen. Bernie Sanders had already charged, in his education plan, that charter schools’ “growth has drained funding from the public school system.” Even Joe Biden—who served under President Obama, an enthusiastic charter supporter—has picked up the refrain. “The...