Advocates of an open border between the U.S. and Mexico do their best to present a mellow American flag-waving image to the public. But when they gather in semiprivate, they communicate much differently to each other. Perhaps they need to be even more careful. In the big pro-immigration marches this spring, Hispanic activists sought to present themselves as "civil rights" advocates in the gentle and inclusive tradition of Rev. Martin Luther King Jr. Oh sure, some of the recent marchers went "off message," carrying Mexican flags and calling for "reconquista," but for the most part, the demonstrators were well-behaved. But...