As political symbols go, it is perfect. An over-sized joke balloon, filled with helium will billow over London, to be buffeted this way and that by the breeze.

The blimp’s creators intend it to mock US President Donald Trump when he visits next week but the embarrassing truth is that it captures more brilliantly the character of the city’s Labour mayor who has allowed this ‘Trump Baby’ balloon to be launched from Parliament Square.

Sadiq Khan: did ever a gassier, more lightweight, plasticky figure dawdle towards the top of British politics?

Once he had so much going for him. Born in 1970 into a family of first-generation Pakistani immigrants, he was taught from earliest days to be aspirational, taking a Saturday job at the Peter Jones department store and working as a labourer on a building site.

London Mayor Sadiq Khan (pictured) and U.S. President Donald Trump have engaged in a long-running war of words over issues such as crime and terrorism

He attended a boxing gym to improve his weedy physique and to learn how to dodge punches. Useful in politics, that is. He read law at university and became a solicitor. He had a good brain; but there was also a hankering for wider acclaim. Humdrum legal work, attending to conveyancing or wills or marital disputes, will not grab headlines the way a big yellow balloon can. Human rights was where the political juice was. The eager Khan was soon developing a speciality in minority-grievance cases.

He became a partner in his firm by his mid-30s, dabbled in local politics and increasingly styled himself the representative of high-profile victims of what he might in those days have called ‘the establishment system’. From there, in those days of early Blairism, it was a comparatively easy step to become Labour MP for Tooting in 2005.

At first he was impressive: his energy palpable as he caught the Speaker’s eye. His voice had an urgency. He worked his fringe into a little quiff with hair gel. He was a boxy-shouldered, bustling figure and earned some respect for the way he ignored the New Labour Whips and – apparently acting from the highest principles – opposed Tony Blair’s plans to lock up terror suspects for 90 days without charge. He also criticised Blair for the invasion of Iraq.

But then ambition again infected his bones. After Gordon Brown replaced Blair as Prime Minister, the Whips sidled up to the man who had so recently rebelled against them.

Did he fancy joining their crew? A job in the Government! Mr Khan almost bit their hands off and became a Whip (ie an enforcer among Labour MPs). Soon he was badgering other MPs to vote for detention periods of 42 days without charge. How easily had those once precious principles been blunted.

Since then this oddly insubstantial figure has floated upwards – and away from his working-class roots – on a series of felicitous thermals. Most politicians are egomaniacs but this one takes it to new, almost comical extremes (there is a glorious photograph of him striking a butch pose and placing his hand on some railings which had a ‘WET PAINT’ sign on them).

Most politicians break or bend their word but this one does it with bells on. Most politicians are opportunists but Sadiq Khan has made short-term posturing so much his speciality, he would win the gold medal for it at any Interstellar Olympics. Take his non-gender-specific traffic lights. When ‘trans rights’ were the big thing a couple of years ago, someone suggested – for all I know in jest – that it was a denial of trans people’s rights that pedestrian signals at road crossings only showed a little green or red man.

And so the newly-elected Mayor Khan ordained that such lights, at public cost and to widespread mockery, should be altered so that the signals now show politically-correct diagrams rather than a little chap.

Take his indignant demand for a national bank holiday for Prince Harry’s wedding (er, it was held on a Saturday). One day it was a headline, the next day, amid embarrassed coughing, it could be filed under ‘Y’ for ‘yesterday’s news’.

Or his feckless declaration that new homes in central London should not be allocated parking spaces because residents should use public transport – the same public transport whose fees he has allowed to rise – and save the planet by reducing their carbon footprint. How were homeowners meant to struggle home with the weekly supermarket shopping?

An inflatable balloon depicting Donald Trump as a baby wearing a nappy, which will fly over London when the President visits on July 13, with one of its creators Leo Murray

Mr Khan’s eco-lecturing did not stop him making a much-trumpeted vanity trip to Pakistan and India where one of his great photo-opportunities coincided with a terrible smog in Delhi and he was almost completely invisible posing for photographs in the pea-souper. The trip was depicted as a ‘coming home’ for him, yet when he was asked about that, he said, ‘I’m from London, mate’. Was he playing it both ways, tacking to the minority-ethnic vote while at the same time asserting his London-born credentials? Perish the thought!

But that, alas, is the nature of this shallow, self-serving figure, a man many Labour MPs have still not forgiven for encouraging Ed Miliband to stand against his brother David for the Labour leadership in 2010, a disastrous misjudgement which led to the party being taken over by the extremists of Momentum.

Mr Khan may not worry much about that. After all, he himself is safely ensconced as London mayor, having beaten a weak challenge from the Tory candidate Zac Goldsmith in 2016. I’m all right Jack!

Likewise, he will be thinking himself jolly clever this weekend as the country prepares for Mr Trump’s visit. He has got one over the most powerful man in the world!

In London, it is claimed, Donald Trump is widely disliked. He is certainly opposed by the metropolitan elite, who have never forgiven him for beating their heroine Hillary Clinton in the 2016 presidential election.

You wonder, however, what they think of Trump on those building sites and boxing gyms where the young Sadiq Khan spent his time.

I suspect they have a rather higher opinion of the US President as someone who stands up for the working classes and who speaks their language.

Trump is certainly a better communicator than arid, charmless Khan. The best political communicators speak from the heart. It is when politicians strike false poses, with words written for them by strategists and advisers, that they fail to convince.

And the sorry fact is that since he became London mayor, Mr Khan has struggled to say anything interesting or radical.

When the creators of the ‘Baby Trump’ balloon first told London’s City Hall about their stunt, they met with little reaction. But then, suddenly, the project was given the mayor’s blessing.

Did Mr Khan’s Press handlers suddenly get wind of the wheeze and spot its potential for a few headlines that might move the public’s attention away from his dreadful record on crime prevention?

Mr Khan’s decision to make a political thing of this balloon lark will no doubt win the acclaim of the Twittersphere. But the City of London’s banks and finance houses depend greatly on American custom and goodwill. Donald Trump is a notoriously touchy president. As a nation we need to persuade him to give post-Brexit Britain a trade deal.

The balloon stunt will gain Mr Khan oodles of publicity. But what will it do for the reputation of a City, and our country, which needs to earn the goodwill of the White House?

Would a serious mayor, a proper, substantial, internationally-minded leader of our capital city, in touch with his real constituents rather than fringe protesters, not have put such considerations above his own childish posturing?

As for that Baby Trump inflatable, we must earnestly hope it is not peppered by someone with a long-distance rifle. Pop goes the weasel Khan’s silly stunt. One can but hope.