The Obama administration has decided to file suit to block a new Arizona law aimed at deporting illegal immigrants, thrusting itself into the fierce national debate over how the United States enforces immigration policies. The move is a rare instance of the federal government forcefully intervening in a state’s affairs, and it carries significant political risks. With immigration continuing to be a hot-button issue in political campaigns across the country, the Arizona law, which gives local police greater power to check the legal status of people they stop, has become a rallying point for the Tea Party and other conservative...