No more pussyfooting around: Nigel Farage and his associates have poisoned our country’s political culture, and it’s time to push back. Their offensive – in every sense of the word – has been so swift, so devastating, that we risk normalising it.

Let’s take a moment to absorb their latest intervention. In June this year, Jo Cox, a Labour MP, was murdered by Thomas Mair, a fascist, white supremacist terrorist. As he stabbed and shot her, he yelled “Death to traitors, freedom for Britain!” This was a political act: the use of violence to achieve political ends. The individual who politicised Jo’s death was the terrorist who killed her.

If anybody has a right to speak out about the dangers of hatred and extremism in modern Britain, it is Jo’s bereaved husband, Brendan.

On Tuesday morning, hours after a truck was driven into a Berlin Christmas market, Nigel Farage spotted an opportunity. “Terrible news from Berlin but no surprise,” he wrote, not even bothering to separate his horror and his vindication with a comma. “Events like these will be the Merkel legacy.” Brendan Cox’s response – “blaming politicians for the actions of extremists? That’s a slippery slope Nigel” – was logically flawless.

Farage’s response in turn – that Cox “would know more about extremists than me” because of his links to the anti-fascist organisation Hope Not Hate – is shocking on a number of levels. First, he is talking about a widower whose wife was murdered by an extremist six months ago. Second, he smeared an organisation that exists to drive back racism – at a time when hate crimes have surged after a referendum campaign made inflammatory by politicians including Nigel Farage.

Then came Farage’s sidekicks. “When are we allowed to say that Brendan Cox is a total arse?” tweeted the Breitbart columnist James Delingpole. “I’m sorry about his wife but he chose to massively politicise it. Who does that?” asked Ukip bankroller Arron Banks, accusing Jo’s widower – rather than the far-right terrorist who killed her – of politicising her death.

When Hope Not Hate threatened legal action to defend its reputation after being smeared on national radio by a prominent politician, Farage’s henchman Raheem Kassam accused the organisation of “using threats to attempt to silence political opposition. Idiots don’t realise how powerful he became this year”.

When Farage’s power was queried, Kassam responded with a picture of Farage and Donald Trump in the president-elect’s golden lift. Don’t mess with Farage, was Kassam’s implication, because he now has the patronage of the soon-to-be most powerful man on Earth. (You can donate to Hope not Hate’s legal fund here.)

These individuals are a political cesspit. They are almost farcically unpleasant comic-book villains. Kassam has called for Nicola Sturgeon’s mouth to be taped shut, “and her legs so she can’t reproduce”, while Banks has ridiculed journalists for reporting racist incidents. Farage himself stood in front of a poster featuring dark-skinned migrants and the words “BREAKING POINT” just days before the referendum.

Some say: just ignore them. Treat them as internet trolls who thrive on attention. They hate nothing more than being ignored.

This is, I’m afraid, terribly naive. These individuals are transforming political culture in Britain. Consider how much more venomous, poisonous and intolerant politics has become in just the course of the year. Left unchecked, it will get much, much worse.

There is a deliberate attempt to delegitimise all shades of progressive opinion. This is the approach of the rightwing authoritarian populism sweeping the western world: to treat all left-of-centre opinion as illegitimate, extremist and even treasonous. The British press – dominated as it is by rightwing oligarchs – is instrumental in forging this intolerant new culture.

Two months ago, the Daily Mail had a front page that read: “Damn the unpatriotic Bremoaners and their plot to subvert the will of the British people.” The Daily Express opted for “Time to silence EU exit whingers.” As Brexit encounters trouble – as it will – this will deteriorate, and rapidly so. Critics of the government’s Brexit agenda will be transformed into traitors and wreckers responsible for sabotaging Britain’s grand national revolution.

Online, you can see how hysteria and hatred are being stirred up. During the referendum campaign a far-right activist photoshopped a picture of me holding a whiteboard explaining my reasons for voting remain: it was changed to claim it was because “Britain’s loss of independence is justice for colonialism”. It was shared on various far-right websites, including Britain First. Using their own names, people responded by suggesting we “should start just shooting these freaks”, that I was “a traitor and [need] to be erased”, that I needed a “smack in the face with a baseball bat”, and so on.

From politicians to journalists, many of those deemed to be dissenting from the tide of rightwing populism have experienced the same. Next month Donald Trump will enter the White House, becoming the chief patron of the rightwing populists, and there will be many elections in which such movements are expected to succeed, such as in France and the Netherlands.

The atmosphere is intolerant now; the risk is it may become suffocating. And as it does, as more hysteria and hatred is stirred up, violent extremists – such as Jo Cox’s killer – will become ever more emboldened. For some reason we forget that it was only five years ago that the far-right terrorist Anders Breivik marched on to a Norwegian island and murdered dozens of young socialists. Why? Because he considered them to be traitors to their country, responsible for the “Islamisation” of Norway and Europe.

We face a great danger, and not even those who will suffer because of it have realised just how grave it is. Intolerance and hatred have been legitimised across the western world. Dissent is becoming treason. That is bad enough. But there are other violent extremists who are being both radicalised and legitimised across the west. If we don’t take a stand now, new dark chapters are soon to arrive.