In that tumultuous summer of 2015, when Jeremy Corbyn was transformed from “obscure backbench MP with more chance of winning X Factor than becoming Labour leader” to leader-in-waiting, his opponents issued a solemn warning. Conservative Central Headquarters, it was said, had a dossier the size of Canada detailing his links to all sorts of unsavoury, dubious, anti-British individuals, groups and regimes. In a general election campaign, the full arsenal of the “Corbyn Hates Britain” files would be deployed by the media, leaving Labour’s electoral prospects an irradiated wasteland.

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Given this bravado, in the first few weeks of the snap election some Labour MPs were indeed genuinely concerned that, who knows, the Sun was about to plaster its front page with an exclusive image of Corbyn on a Butlins holiday with Isis leader Abu Bakr al-Baghdadi. The tone was hysterical: as polling day approached, the Daily Mail devoted 13 pages to smearing Corbyn and his allies as terrorist sympathisers. It did not have the desired effect. It was a bit like an eccentric raving on a bus about impending Armageddon. The result is all the other passengers uncomfortably looking into their lap: just too OTT to take seriously. Voters looked at their shrinking pay packets, growing debt and the booming wealth of the elite while services were starved of cash – and rewarded Labour with 40% of the vote. At his party’s conference, Corbyn jokingly pleaded with the Daily Mail to make it 28 pages of smears next time.

Which brings me to the current rightwing hysteria and slightly surreal cold war homage: Corbyn meeting a diplomat from Czechoslovakia (which ceased to exist 25 years ago) named Ján Sarkocy in the 1980s who, it turns out, was a spy. According to the Czech security forces archive (ABS), Corbyn did not know he was a spy. Sarkocy has also claimed that he organised Live Aid, which he alleges was apparently funded by Czechoslovakia, and also purports to have known what Margaret Thatcher had for “breakfast, lunch and dinner”, and what clothes she would wear each day.

I am mildly sceptical that Corbyn was best positioned to know Margaret Thatcher’s sartorial choices in advance

I am mildly sceptical that Corbyn was best positioned to know Thatcher’s sartorial choices in advance. All that this crank seems to have gleaned about Corbyn is an antipathy to the Thatcher government and US foreign policy, which he could have worked out by reading Hansard. Corbyn used to be known as the “foreign secretary of the left” in leftwing Labour circles: him meeting foreign diplomats for a chat is about as surprising as the Tories selling arms to brutal dictators to drop on innocent civilians.

This is grossly irresponsible pseudo-journalism, concocting an obsessive, unrelenting media campaign off the back of the rantings of a source with about as much credibility as Dave down the local after one too many sambucas.

Theresa May is crudely attempting to exploit this embarrassing non-story. Tory MP Ben Bradley, best known for being forced to apologise after calling for vasectomies for “unemployed wasters”, was forced to delete a potentially libellous tweet about this concocted story after Corbyn threatened to sue him.

The rightwing press is whipping up hysteria, calling Corbyn a “collaborator”. Today’s ludicrous Telegraph splash is “Corbyn urged to reveal his Stasi file”; the Daily Mail opts for “Time to be open, Comrade Corbyn” (sadly there is no free pullout on how to look for reds under your bed). On the one hand, this is quite a cute throwback to cold war smears against Labour leaders, nearly three decades after the Berlin Wall fell. In 1924, MI6 forged a letter from Soviet leader Grigori Zinoviev urging subversion on the part of pro-Communist forces in the Labour party: the scandal helped ensure a decisive Tory victory. Harold Wilson was continually smeared as a Soviet spy, and there were even murmurings of a rightwing military coup against him. “KGB: Michael Foot was our agent” was one Sunday Times splash (leading to the paper being forced to pay libel damages in 1995); Neil Kinnock has been accused of colluding with the Soviets, and in 1992, a Sunday Times splash simply read: “Official: Kinnock’s Kremlin Connection”.

But here’s why it is sinister. Ever since Corbyn became leader, there has been a systematic attempt to strip away Labour’s democratic legitimacy. The Labour leadership has been relentlessly portrayed as Britain-hating terrorist sympathisers, as potentially mortal threats to the nation and its security. When the election was called, the Sun front page demanded “Blue murder” to “kill off Labour”, while the Daily Mail demanded that May “Crush the saboteurs”. In the post-Brexit political landscape, of course, the mainstream press even labels judges as “Enemies of the people”. In their efforts to smear Labour, and delegitimise them as an acceptable political force, British Toryism is destroying democratic culture in this country.

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And then there is another, more chilling, element too. On the eve of the EU referendum, a fascist terrorist murdered Labour MP Jo Cox, giving his name in court as “Death to traitors, freedom for Britain”. This month, anti-Muslim terrorist Darren Osborne was sentenced to life for driving his van into a mosque: radicalised partly by the media, he admitted to originally hoping to murder Jeremy Corbyn, because it would mean “it would be one less terrorist [on] our streets”. Anyone on the left with a public position can testify to receiving with threats of violence attached to accusations of “treachery”.

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When it comes to foreign policy, it is the Tories with questions to answer: from arming the extremism-exporting headchoppers of Saudi Arabia as they bomb Yemen’s children to selling fighter jets to the Turkish regime. It is all too convenient for them to deflect. And no one is saying that Corbyn and Labour should not face scrutiny. But this is not “scrutiny”. This is a dangerous game indeed, which undermines British democracy and legitimises far-right extremists. Both the Tories and their media allies are fuelling the radicalisation of the right. And if things turn ugly again, they will have very profound questions to answer.

• Owen Jones is a Guardian columnist