Pentagon procurement scandals are not usually material for presidential campaigns. But John McCain, the Arizona Republican senator, has touted his role in exposing one of the biggest military acquisition scandals, writes Demetri Sevastopulo .

In 2002, the air force awarded Boeing a sole-source contract worth $23.5bn to lease air-to-air refuelling tankers. The deal came under scrutiny on Capitol Hill amid allegations that it was an attempt to help keep Boeing's ailing 767 production line open.