Like free trade and the size of government, immigration is one of those bone-deep American divides, something we've been arguing about among ourselves for centuries. Now a new Arizona law has deepened the split. The law authorizes local police to check papers and arrest undocumented residents. Critics compare it to apartheid, to a police state, to Nazis singling out Jews. Supporters say it simply puts some teeth into gummy federal immigration laws and backs up the porous border fence. "When the federal government failed to act, Arizonans did," says J.D. Hayworth, a Republican running for the U.S. Senate. But with court challenges threatened, police officials opposed to their new role and the mayor of Phoenix worrying about potential damage to the economy, chances are good that the law will turn out to be more of a vote polarizer than a problem solver. The sun may shine equally on both sides of the border, but the same is not true of peace and prosperity. As long as those blessings are abundant in the U.S., they will be mighty magnets.