On an afternoon in Sacramento, Calif., 147 vehicles are parked outside a theater complex under a hazy spring sky. There are shiny Town Cars and rusty Ford pickup trucks, perky Rav 4s and burly Silverados. For all their differences in name and prestige, they have one thing in common: Their backsides are pristine. Not one of them sports a single gem of wisdom plastered to a bumper. Bumper stickers, first used in election campaigns after World War II and once a powerful form of political and cultural expression, can still be found in trinket shops and Internet stores. But fewer...
