NEARLY ALL THE Republican presidential hopefuls are now on record as wanting to build a 2,000-mile-long fence on the U.S. border with Mexico — a wildly expensive, staggeringly wasteful project that would do little to deter illegal immigration. The candidates have coalesced around this chimera even though illegal crossings, as measured by apprehensions by U.S. Border Patrol along the Southwestern frontier, are at their lowest level in 40 years.

Consider the profligacy. About a third of the Mexican border has been fenced, and the price tag for finishing the remaining 1,400 miles or so is variously estimated at $22 billion (the New York Times); $30 billion (Texas Gov. Rick Perry); or much more, according to government projections based on maintenance and repairs over the fence’s 25-year life expectancy. In general, those estimates exclude the cost of land acquisition, as well as the potentially severe environmental damage that fencing would inflict in some areas.

Now consider effectiveness. Last year, U.S. officials reported more than 4,000 breaches of the existing 650 miles of fencing. A video posted on YouTube illustrates the vulnerability, showing two young women scaling the fence in less than 18 seconds. And the Border Patrol considers hundreds of miles of frontier to be virtually impassable as it is, owing to remote, difficult terrain and deep river canyons.

Mr. Perry, a border-state governor, grasps the futility of trying to barricade every mile. Lately, though, he has played to immigration hard-liners by saying he’d “shut down” the border within 12 months of taking office, whatever that means. He’s also distinguished himself as a flip-flop artist, telling a New Hampshire crowd Tuesday that he would deport every illegal immigrant detained in the country. Never mind that he said in September that it was heartless to deny opportunity to illegal immigrants who grow up in the United States.

Perhaps Mr. Perry was trying not to be outdone by Rep. Michele Bachmann (R-Minn.), who says she’d deport all 11 million illegal immigrants, in stages. Presumably, that includes the 8 million who make up more than 5 percent of the U.S. work force, meaning Ms. Bachmann would likely cripple the nation’s hospitality industry and trigger a food crisis by robbing farms and processing plants of workers.

Even Newt Gingrich, who has rejected mass deportations in favor of a “humane” approach to illegal immigrants, has shifted into get-tough mode. Although he still says he’d let longtime undocumented immigrants remain in the country, he wooed a conservative crowd in South Carolina by embracing the state’s lawthat authorizes officials to check the immigration status of anyone suspected of lacking proper documentation, saying he’d complete the border fence within a year of taking office, and pledging to cut funding to cities that don’t enforce federal immigration laws.

In the meantime, back on Planet Earth, the reality is that illegal border crossings have plummeted. This is a result of a beefed-up presence of U.S. forces on the Mexican border; the recession’s impact on U.S. labor markets; and better educational opportunities and plunging birth rates in Mexico. Undocumented immigrants continue to live in the shadows, doing hard, dirty and dangerous work that most Americans won’t do, all the while paying taxes and contributing to the economy. The plain fact is that the United States and undocumented residents are mutually dependent, and no amount of Republican campaign rhetoric is going to change that.

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