By Jonathan Freedland
Barack Obama says the most important quality is vision for the future. No, says John McCain, the key requirement is experience - or at least that's what he said until he picked Sarah Palin as his running mate.
Both want the most powerful job in the world - but neither they, nor anyone else, can agree on what, precisely, are the qualities needed to serve as president of the United States.
Indeed, there is not even a job description - only an oath of office demanding the president defend the US constitution.
What's more, the job keeps changing, evolving constantly in the 230 years since the founding of the republic.
Still, an understanding has gradually emerged of the key qualities required of a president.
The trouble is, they are so many and various, it's almost impossible to imagine any normal human being matching up.
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