Every four years, despite ample evidence to the contrary, the US celebrates the myth of presidential omnipotence – of the office, at least, if not its occupant. The country is looking for the one man or woman who can do the biggest job in the world, take the 3am phone calls and use those awesome powers to set to rights all that is wrong, from the war on terror to indiscipline in schools, from economic inequality to the state of the roads. It is a cherished illusion. In 2008, the worst financial crisis since the 1930s has shattered it before the new president is even in the job.