CALDWELL, IDAHO - This oasis of irrigated farm country in the high desert is a long way from the US-Mexico border, and even farther from the nation's capital, but it represents America's new battleground on immigration policy. Here county commissioner Robert Vasquez is trying to do what he says Washington won't: crack down on illegal immigration. He recently sued several local employers in a novel bid to use federal anticorruption law to prevent hiring illegals. Mr. Vasquez's controversial crusade is part of a larger pattern. As the border continues to be punctured by illicit crossings, and as immigrants spread to...