On 27 April 2015 a white van was reported stolen in east London – inconvenient for the owner, but not politically significant. The theft only made the news because its cargo included 200,000 blank ballot papers, due for dispatch to Hastings and Eastbourne, in East Sussex, ahead of the general election the following week. Police believed that the vehicle was the target, and that the crime was not part of some larcenous automotive vote-rigging exercise.

It was a non-event. It wasn’t even the best white-van-related story of the last parliament. I had forgotten all about it until a few weeks ago when a resident of Eastbourne – a disappointed Labour supporter, a young man showing no outward symptoms of crackpottery – cited it as evidence that the election was fixed: “I don’t believe the Tories won,” he told me. (Historical note: the Tories won.) Opinion polls said one thing; results said another. It was all a bit mysterious until you considered the van, he added. “The thing is, I’m a bit of a conspiracy theorist.” He made it sound like a professional qualification.

It is hard to measure paranoia but there is a lot of it about and – perhaps I am being paranoid here – it seems to be acquiring a more assertive character. Suspicion of politicians is not new. But I see more joining the dots of suspicion into elaborate constellations.

I have met Ukip supporters who believe that the Labour party is a vast paedophile protection racket. That is before they even get to European plans for the genetic dilution of Britain’s indigenous population. The Scottish independence referendum spawned a multiverse of parallel realities, featuring secret oil fields, stolen ballot boxes and collusion between the BBC, the Bank of England and Buckingham Palace to procure a false result on polling day.

There have been enough reports in recent weeks of Labour councillors and candidates peddling antisemitic mumbo-jumbo – Jews as puppet-masters behind 9/11, Isis and global capitalism – to suggest the party has an infestation on its hands. I must tread carefully here. I do not want to accuse everyone who yearns for an independent Scotland – or wants Britain to leave the EU, or voted for Jeremy Corbyn to become Labour leader – of indulging fanatics, let alone forming some combined assault on rational discourse. Millions of reasonable people support those propositions and history might vindicate their judgement.

Those just happen to be the movements that have recently mined rich seams of social, cultural and economic alienation from the political centre, which they locate geographically in Westminster and which they define ideologically as soulless technocracy fiddling at the margins of globalisation.

Unrelated insurgencies can share a common idiom of dissent: rejection of “mainstream media” as the propaganda tools of a self-serving elite; refusal to engage with evidence if the source is ideologically impure; inability to distinguish between reporting of news and malicious spreading of smears.

Those are also the components of a conspiracy theory: the self-reinforcing belief that attack and ridicule are precisely the weapons that rattled conspirators would use to sustain their conspiracy. Faith in the original proposition is strengthened when the establishment is depicted closing ranks to silence those who dare to speak out.

What if the fringe encroaches inwards to the centre and the mainstream shrinks in proportion?

There is one place in particular where conspiracy theory and mainstream opposition politics cross-fertilise. It is the assumption that ministers spend their time lying because they dare not reveal their wicked true motives. You don’t have to stray far along the spectrum of left opinion, for example, to encounter the view that Tories are evil. Not just incompetent or misguided, but thirsting for cruelty. Jeremy Hunt is presumed to wake up of a morning pondering ways to destroy the NHS, while Nicky Morgan masterminds the privatisation of childhood. Their public personas are masks that hide those ambitions.

Few of us know anyone so systematically duplicitous or vindictive. It is possible that public office attracts sociopathic personality types. It seems more likely that most MPs come to the job with honest intentions and that unhappy outcomes are best explained by the mundane forces of stupidity and bad luck. Everyone who has worked in government reports that conspiracies are far too difficult to organise, that civil servants would bungle their implementation anyway, and that a lot more effort is spent trying to attract voters’ attention to what ministers are doing than in throwing them off the scent.

The presumption that politicians are always up to something more sinister is a marker of what used to be the political fringe. That word seems inadequate now. It denotes a place on a spectrum that is, by definition, narrow and self-limiting. The fringe must be smaller than the mainstream to whose edges it is confined. But what if the fringe encroaches inwards on the centre, and the mainstream shrinks in proportion? What if “fringe” politics denotes not a place but a habit of mind – a way of explaining the world, a set of rhetorical barricades against contradiction? The fringe can then overwhelm the mainstream.

For a taste of that, we can look across the Atlantic, at Donald Trump’s march on the Republican presidential nomination. Here is a party that spent years campaigning against the morals and motives of its rivals, only to discover that it cannot control the audience it nurtured for that brand of politics. Trump was an avid promoter of the “birther” proposition that President Obama was foreign-born, probably a secret Muslim, and so alien to the White House. Trump’s rise erases a boundary between whackjob conspiracy theory and legitimate politics.

The US and British systems are only superficially alike. The shared language obscures vast cultural and institutional differences. We do not have the same conservative traditions of bellicose illiberalism and messianic political religiosity. Nor do we have the same cult of wealthy bombast. It is hard to imagine a character with the sheer vulgarity of Trump appealing to a UK electorate.

We tell ourselves that we are smarter, more civilised, less credulous and worldlier than our American cousins. We want to congratulate ourselves on the certainty that it could not happen here, denying the possibility that, in an understated, low-key, parochial and idiosyncratically British kind of way, maybe it already has.