CHICAGO — The title of the nation’s largest labor union — the National Education Association — seems calculated to blur the fact that it is a teachers union. In this blunt city, however, the teachers union candidly calls itself the Chicago Teachers Union. Its office is in the Merchandise Mart, a gigantic architectural Stonehenge, which resembles a fortress located on the Chicago River, which resembles a moat. Which is appropriate.

Unions are besieged, especially public-sector unions, particularly teachers unions, and nowhere more than here. Teachers unions have been bombarded with bad publicity, much of it earned, including the movie “Waiting for ‘Superman,’ ” and have courted trouble by cashing in on sentimentality, cloaking every acquisitive demand in gauzy rhetoric about how everything is “for the children.”

Still, have sympathy for Karen Lewis, 58, a Dartmouth graduate who is a daughter of two African-American teachers. She taught chemistry for 22 years until becoming president of the 26,502-member CTU. Her job is to make life better for her members, not to make life easier for Mayor Rahm Emanuel, with his roughneck’s reputation and stevedore’s profanity, whose ideas are as admirable as his manners are deplorable.

He thinks that improved schools, including more charter schools, might arrest the exodus to the suburbs of parents whose children are ready for high school, so he wants a longer school year and school day. America’s school year (about 180 days) is one of the shortest in the industrial world, and while middleclass children may leaven their summers with strolls through the Louvre, less privileged children experience “summer learning loss.” Remediation requires the first few weeks of the fall term, which effectively further shortens the school year. And Chicago’s school day is the shortest of any large American district.

The CTU wants a pay raise — 30 percent — proportional to Emanuel’s 90-minute increase in the school day and 10-day increase in the school year. He has canceled a 4 percent raise and offers only 2 percent. He says benefits the CTU has won — e.g., many teachers pay nothing toward generous pensions they can collect at age 60 — could in just three years force property taxes up 150 percent and require classes with 55 students.

Even discounting Emanuelean hyperbole, whose fault is this? Just as foggy rhetoric about corporations’ “social responsibilities” obscures the fact that a corporation’s responsibility is to maximize shareholder value, blaming unions for improvident contracts ignores the fact that a union’s principal task is to enhance members’ well-being — wages, benefits, working conditions. Unions can wound themselves by injuring their industries (e.g., steel and autos), but primary blame for improvident contracts with public employees belongs to the elected public officials who grant them.

Anyway, money — salaries and pensions — may not be the most problematic point of contention. It might be teacher “accountability,” including merit pay, and identifying failing schools and teachers. Lewis says, “We can’t choose the children that come into our classrooms.” Chicago schools are 86 percent black and Hispanic, and low pupil performances strongly correlate with household incomes.

Teachers unions, however, have painted themselves into a corner by insisting that spending is the best predictor of educational performance — increase financial inputs and cognitive outputs will rise. In the last 50 years, real per pupil spending nationwide has tripled and the number of pupils per teacher has declined by a third, yet educational attainments have fallen. Abundant data demonstrate that the vast majority of differences in schools’ performances can be explained by qualities of the families from which the children come to school: The amount of homework done at home, the quantity and quality of reading material in the home, the amount of television watched in the home and, much the most important variable, the number of parents in the home. In Chicago, 84 percent of African-American children and 57 percent of Hispanic children are born to unmarried women.

The city is experiencing an epidemic of youth violence — a 38 percent surge in the homicide rate, 53 people shot on a recent weekend, random attacks by roving youth mobs. Social regression, driven by family disintegration, means schools where teaching is necessarily subordinated to the arduous task of maintaining minimal order.

Emanuel got state law changed to require unions to get 75 percent of the entire membership rather than a simple majority to authorize a strike. Some people thought this would make strikes impossible. The CTU got 90 percent to authorize. Lewis’ members are annoyed, and are not all wrong.

George Will’s column appears Thursdays and some Sundays. Reach him at georgewill@washpost.com.