My friend E. J. Dionne Jr., a liberal columnist for the Washington Post, is a fine man with, I feel safe in asserting, a warm heart. But he betrays in a recent column a persistent failing of the Left — imperviousness to evidence. Describing Speaker Boehner’s tactics in the budget fights with Democrats, Dionne wrote: Begin with the outrageous $1.1 billion, 15 percent cut from Head Start, a program that offers preschool education to roughly 965,000 poor children. According to...