Not to get all Clintonian, but answering this question depends on the meaning of "a fence." Do we need a continuous fence for the whole 2,000 miles from San Diego to the mouth of the Rio Grande? Probably not, though I'd never say never. But we do need fencing along large parts of the border, and better fencing than we have now.
Currently, about 650 miles of our border with Mexico is fenced, though about 300 miles of that is just vehicle barriers, or what the Border Patrol calls "Normandy barriers", which prevent trucks from driving across but not people. Your grandma could hop over them--heck, I've hopped over them.
Where we do fencing we need to do it right. A double line of fencing is what's needed, with a road for patrolling in between. This slows down even the most committed illegal crosser and unfortunately, only about 1 percent of the border has a barrier like this, most of it south of San Diego. Such a barrier proved itself there, with crossings declining dramatically and order restored in what were chaotic border neighborhoods.
But a fence is just another tool used by the Border Patrol, like a helicopter or a flashlight. It will only "discourage illegal immigration" if it's used in combination with a range of other tools and techniques. Hidden sensors and forward operating bases for agents, for example, but also an expansion of what's dubbed Operation Streamline, where border jumpers are actually prosecuted and jailed for a few weeks before being dumped back across the border. In addition, we need to turn off the magnet of jobs that induces people to cross the border in the first place; the most important step here is to mandate use of the online E-Verify system for all hiring, so employers will know if the Social Security number a new employee is giving them is phony or stolen.
In short, fencing is an essential part of a broad, multilayered approach to controlling our southern border. But it's not a silver bullet and you can't just build it and forget it.
About Mark Krikorian Author of 'The New Case Against Immigration, Both Legal and Illegal' and 'How Obama is Transforming America Through Immigration'
Donna Wiesner Keene Washington Liaison for TheTeaParty.net