Imagine if our national economy, culture, and politics were rooted in the idea that the default American household is white and Christian. There would be no Spanish-language campaign ads and TV shows, no interracial families depicted in commercials, no fill-in-the-blank Heritage Day at ballparks. Workplaces would see no need to accommodate holiday schedules for Muslims or Jews. That was a good bet more than 50 years ago, when the country was 88 percent white and 90 percent Christian , and less