James G. Gimpel is a professor of government at the University of Maryland, College Park. He can be reached at jgimpel@gvpt.umd.edu. This Backgrounder examines the political implications of large-scale immigration. Between 1980 and 2008, 25.2 million people were granted permanent residency (green cards) by the United States. A comparison of voting patterns in presidential elections across counties over the last three decades shows that large-scale immigration has caused a steady drop in presidential Republican vote shares throughout the country. Once politically marginal counties are now safely Democratic due to the propensity of immigrants, especially Latinos, to identify and vote Democratic....