ASK any Republican presidential candidate, and they will tell you without hesitation: America’s border with Mexico is as leaky as a sieve. Mitt Romney thinks all 1,969 miles (3,169km) of it must be fenced. Michele Bachmann wants a double fence. Rick Perry was pilloried for suggesting that in some rugged areas, more “aviation assets in the ground” might be better than fencing. Bemused by such talk, Barack Obama joked earlier this year that Republicans would not be happy until there was a moat full of alligators to keep illegal immigrants at bay. A few months later Herman Cain said there should be an electrified fence, with a charge strong enough to kill. He later explained that he too was joking, but would never apologise for standing up for America.