As the writers of the TV show Happy Days approached their fifth season they were running out of ideas for storylines. So, in the season premiere, they sent the Fonz to Los Angeles where, in a bid to prove his bravery, he put on a pair of water skis and jumped over a shark.

That moment spawned a phrase — “jumping the shark” — which is used to describe the moment when something is taken too far, loses all credibility and makes everyone involved look silly.

In recent weeks, the Brexit campaign has jumped the shark.

Unsurprisingly it was Boris Johnson — a man who would happily jump over a shark in the Thames if he felt it would get him closer to 10 Downing Street — who started the ball rolling by reacting to Barack Obama’s assertion that Britain is better off in the European Union by calling him a “part-Kenyan President” and claiming his views are informed by an ancestral disdain for the British Empire.

He soon followed this with an even more bizarre claim that the European Union was in some way of a piece with Hitler’s plans for Europe, as if the union built to ensure peace across the continent after the world wars was in any way comparable to the genocide that preceded it. His claim that the EU stops people selling bananas in bunches of more than three — I’ve just bought a bunch of six — seems somewhat tame in comparison.

But Boris is not the only one at it. Nigel Farage, never one to be outdone in the outrage stakes, recently said he understood why some people might consider “violence is the next step” if the referendum is lost.

And just yesterday you had Penny Mordaunt, the armed forces minister no less, brazenly claiming that Britain wouldn’t be able to veto Turkey joining the EU, despite the fact it is emphatically not true.

Dominic Cummings, a senior figure in Vote Leave, has suggested that those who believe we should remain in the EU are like the appeasers of the 1930s. Wearing the slightly crazed look of someone who jumps sharks for a living, Cummings told the Commons Treasury Committee that the “conventional wisdom” of today is as misguided as it was then. The fixation with the Nazis among Brexiteers is as historically illiterate as it is revolting.

And then there are the comic-book conspiracy theories. Priti Patel, a Cabinet minister, explained that the reason the head of the International Monetary Fund, Christine Lagarde, had warned of risks to the UK economy from Brexit was because George Osborne had asked her to “bully the British people”.

Cummings has asserted that the Cabinet Secretary, Jeremy Heywood, is running an intimidation scam out of the Cabinet Office, threatening people to toe a pro-European line. I saw the Cabinet Office at work for five years. It is a slightly herbivorous part of the government machine. The notion that it is the Whitehall equivalent of the Sopranos is laughable.

So why the bizarre claims, hysterical over-reactions and unsubstantiated conspiracy theories? Simple: the Brexit campaign has run out of rational arguments.

The truth is, there are a number of rational criticisms that can be made of the EU — most especially the acute economic and social imbalances within Europe — but those criticisms do not amount to a rational case for exit.

I am highly critical of much of the overcentralised and unaccountable way in which Whitehall and Westminster operate — unelected peers, big money funding parties, a lopsided electoral system and so on. But I would never advocate razing them to the ground as a solution.

All political institutions are imperfect. Politics is about improving what we have. Religion aspires to perfection but politics is about making things better, not making things perfect.

That is why the Brexiteers are sounding increasingly zany: their belief that life’s imperfections will all be cured if only we quit Europe is a kind of theology, an unquestioning leap of faith into the unknown.

They cannot tell you what will happen on June 24 if we vote to leave. They cannot tell you on what terms our companies will be able to trade with our European neighbours, or anyone else for that matter. They cannot tell you what that will mean for the jobs that rely on that trade. They cannot tell you how we will rise to the biggest challenges — from economic globalisation to climate change to terrorism and cross-border crime — when we cannot work with our neighbours in the same way. They cannot tell you how Britain will influence the rest of the world when we have ceded our influence in our own back yard.

But perhaps the most unattractive aspect of this unblinking belief in their own cause, is that Brexiteers don’t seem to care about the impact on other people’s lives.

'So why the bizarre claims? Simple: the Brexit campaign has run out of rational arguments' Nich Clegg

It’s all very well for Peter Hargreaves, a billionaire Brexit backer, to declare that if we quit the EU “we will be insecure again. And insecurity is fantastic”. Millionaire Leave.EU founder Arron Banks described the loss of thousands of pounds of income per household if we left the EU as “a bargain-basement price” which would be “worth paying”. He might find it a price worth paying, but it will be paid by poorer households, not people like him.

Boris Johnson’s chief economic adviser (who favours Brexit) has warned that “leaving the EU would be an economic shock”. Nigel Lawson has loftily dismissed the higher taxes on UK exports if we left the EU as “trivial”. But there’s nothing “trivial” about losing your job.

And that’s the problem when people become so sure of their cause — against all evidence and logic — they start to assert that they are right even if the consequences for normal people are bad. The needs of normal people are soon sacrificed on the altar of unwavering belief that their way is the only way.

Boris Johnson, Michael Gove and Nigel Farage might be treating this campaign like an Oxford debating club, trying to bluster their way to victory. But this is not a game. They are not the ones who will have to live with the consequences. It’s not their livelihoods at stake.