Boris Johnson
Conservative MP Boris Johnson is part of the Vote Leave Brexit campaign Getty

Former Mayor of London Boris Johnson is, by some considerable margin, the most overrated politician in the UK. I make the assertion fully aware of his enviable talents. I am also one of the few political journalists who detect sincerity as much as personal ambition in his belated, tortured decision to campaign for the UK to leave the European Union.

It is the awesome context in which Johnson makes his provocative, headline-grabbing moves that makes him overrated. He leads the 'Out' campaign as the UK speeds towards a referendum of historic significance. As he does so, there is hourly speculation that within months Johnson could be the next leader of his party, and therefore also be prime minister. This is politics at its most giddying.

Now let us consider Johnson's experience as a senior figure on the national political stage. He has never been a minister, let alone a member of the cabinet. He was fleetingly on the front bench in opposition when Michael Howard was leader of his party, but not for very long before he was forced to resign.

He has been Mayor of London for two terms – a considerable achievement, but still a limited one.

Ken Livingstone, who also served two terms as Mayor, once observed that while London would have him as its leader, he would never have been accepted as a prime minister.

In making the observation, Livingstone displayed a self-awareness that has deserted him since. London's mayoral campaigns, and the job itself, are subjected to much less intense scrutiny than their equivalent at a national level. The demands on the national stage are unrecognisably different and on a grander scale.

Long service on the front bench can be stifling, but is also formative. In particular senior cabinet ministers, from the prime minister downwards, must frame messages bound by collective responsibility except of course in this volatile referendum campaign. They have no choice but to be briefed in meticulous detail about every government policy before conducting long media interviews. They are tested around the clock by the effectiveness of their policy implementation, performance in parliament and beyond.

Because he is one of the most famous politicians in the land, his lack of readiness for the long haul is obscured or ignored

In contrast to these rigorous demands, Johnson embarks on his historic campaign largely untested. Because he is one of the most famous politicians in the land, his lack of readiness for the long haul is obscured or ignored. Fame has been mistaken for experience. The former Mayor is an elegant writer who can range widely. He is proven as a communicator who engages with voters. Above all, he recognises the power of wit as a powerful political tool. In recent decades, UK politics has become largely humourless. Voters respond to humour.

Such qualifications are important, but relatively puny. They explain why Johnson is making mistakes in the referendum. The headlines he has generated linking the EU with Hitler are only the latest example. Johnson was making a wider point about the EU becoming a super-state. As a journalist, he would have known that the reference to Hitler would be the one that captured the attention. He chose to obscure his over-the-top but valid argument about the dangers of an EU superstate by triggering another bonkers 'Hitler' controversy.

Johnson is playing for stakes that are too high at this stage in his career. His hero, Churchill, had been a cabinet minister in times of peace and war before he reached Number 10. Boris has been a Mayor of London with limited powers.

The zany referendum campaign has highlighted and heightened the intense division within the Conservative Party over Europe. The intensity will not subside after the vote in June. It has also exposed the limits of Boris. He does not seem ready to lead the 'Out-ers' with persuasive, disciplined and forensic force or to become the next prime minister at a time of heightened trauma within his party and beyond.

Steve Richards is a political columnist who has written for the Guardian, the Independent, The Times, The New Statesman and Spectator. He presents BBC Radio 4's Week in Westminster and tours with a one-man show Rock N Roll Politics. His next book, Not One Of Us: The Rise of the Outsiders, is to be published by Atlantic