Immigration has a fantastically complicated political history in the United States. It has produced enough populist anger to elect Know Nothing mayors of Chicago, Philadelphia, Washington and San Francisco, all in the 1850s and, more recently, to help Lou Dobbs reinvent his television career and become a best-selling author. But when national politicians have tried to seize on such anger, they have usually failed — and failed quickly. “While immigration has always roiled large sections of the electorate,” said Eric Rauchway, a historian at the University of California, Davis, “it has never been the basis for a national election, one way or the other.”