Stephanie Rawlings-Blake, mayor of Baltimore, almost gets teary eyed when she talks about the need to rebuild her city with immigrants and refugees. It is an issue that is close to her heart, but the argument she uses is not new. It’s the same line that is touted by the mayors of other cities, mostly in the Rust Belt, like Baltimore, Minneapolis, Detroit and Pittsburgh. They crave a steady flow of new immigrants and refugees, they say, because their economies depend on it. Too many of the native-born middle class whites have long since departed for the suburbs. Baltimore, for...