He is, after all, eligible. Johnson was born in New York City in 1964 and holds duel British-American citizenship and has renewed his US passport, which he once threatened to renounce after being barred from using his British passport to change planes in Texas.
In 2006 Johnson - who has also been tipped to become British Prime Minister one day - wrote in The Spectator: "When the going has got tough in England it has sometimes crossed my mind that I could yet activate the Schwarzenegger option and flee to the land of opportunity, perhaps beginning as a short-order chef in Miami before winding up as Colorado senator and, inevitably, president."
In fact, it's entirely possible that, when no longer London Mayor, he could first be UK Prime Minister and THEN be US President:
Who, What, Why: Could Boris Johnson be UK PM and then be US president?
A collection of cultural artefacts
12 May 2014
Boris Johnson has renewed his US passport. It might seem fanciful but could he possibly become UK prime minister and then US president, asks Tom de Castella.
Boris Johnson has renewed his US passport, the Sunday Times reported at the weekend. The news came as a surprise to some.
In a column for the Spectator in 2006 he said he was renouncing his US citizenship after being barred from using his British passport to change planes in Texas. But it appears he didn't follow through.
There has been speculation Johnson might eventually succeed David Cameron as Tory leader. Having dual citizenship is no bar to becoming prime minister. The only requirements are to be able to command the confidence of the House of Commons and be invited by the Crown to form a government, says John Curtice, professor of politics at Strathclyde University. You don't even need to have British citizenship to be an MP. The rules say only that you have to be a citizen of the UK, the Republic of Ireland or the Commonwealth, and over 18.
The US passport revelation leaves an intriguing question floating in the air. Could Johnson be prime minister and then US president? He was born in New York and being born in the US is a constitutional requirement for any presidential candidate. He told David Letterman in 2012 that he could "technically speaking" be US president. In Johnson's 2006 Spectator piece he wrote: "When the going has got tough in England it has sometimes crossed my mind that I could yet activate the Schwarzenegger option and flee to the land of opportunity, perhaps beginning as a short-order chef in Miami before winding up as Colorado senator and, inevitably, president."
Tongue was firmly in cheek. Or was it?
Arnold Schwarzenegger - at one point a popular governor of California - cannot become president because he was born in Austria.
Rumour has it he has in the past lobbied to get the rule changed. But Johnson has no such impediment.
Austrian-born Arnold Schwarzenegger was governor of California - but can't run for US president, unlike Boris
Article II, section 1 of the US Constitution has just three eligibility criteria for being president - being "a natural born American", aged over 35 and living in the US for 14 years. Johnson falls down on the last - but in theory he could leave Downing Street and move to the US and 14 years later enter the race for the White House.
There is nothing in the constitution to stop a foreign leader becoming US head of state, says Dr Joshua Simon, lecturer in American politics at King's College London.
But that's the legal situation. The political one would be rather different.
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