If a malign power wanted to push a peaceful country into political violence, it would follow the example of the British Conservative party. For violence to come from the fringe to the mainstream two conditions must be met: the taboos against violence must be undermined and the forces of order weakened. The Tories have met them both.

Political violence needs authorisation from above. Politicians, intellectuals and religious leaders either issue a call to arms or, more usually, find reasons to “understand” and excuse. In the manner of the sexist judge saying women in miniskirts are “asking for it”, they divert attention from perpetrators and say victims have no one to blame but themselves.

The right once condemned the left for “whatabouttery”. Now Dominic Cummings tells MPs who are receiving death and rape threats that it was “not surprising some people are angry”. A member of Boris Johnson’s cabinet hid behind the coward’s cloak of anonymity and told the Times that Britain would have a “violent, popular uprising” if Remain won a second referendum. Both invoked violence as a tactic to frighten opponents into line.

Cameron’s curse on Britain was to allow a vote to leave without defining what Brexit meant

The point is rarely made on these pages, so let me state it bluntly. People who voted Leave in 2016 have a right to be angry. If you backed Remain and Remain had won, but politicians were still intent on taking Britain out of the EU, you would be angry too.

But Leave anger is based on a sly rewriting of history. Boris Johnson is refashioning the Tory party into an English nationalist party. We won, his message goes. Parliament voted to trigger article 50. MPs promised to deliver Brexit, and now they are betraying it. Demagoguery is the last, best hope of a desperate PM.

The Brexit stab-in-the-back myth deceives because it can never admit that the 2016 referendum was bound to be betrayed. David Cameron’s curse on Britain was to allow the British to vote for Brexit without defining what Brexit meant. Cummings’s Vote Leave was grateful for the free pass. It too refused to say what the public was voting for, because Cummings, Michael Gove and Johnson wanted no part of an “unwinnable” debate. In any case, as Cummings said, “Eurosceptic groups have been divided for years” about what Brexit meant – as the British have learnt to our cost.

The Conservatives created a country in which any form of Brexit would be a betrayal. Politicians who are today conjuring up popular uprisings and accusing their colleagues of breaking their word helped vote down Theresa May’s Brexit deal. Can we honour the referendum and stay in the single market? Or the customs union? There is no right answer to a question that was not posed in 2016.

Even if Nigel Farage, Cummings and Johnson crash us out, workers who lose jobs and patients who are denied medicines will be able to claim they have been betrayed because the 2016 referendum did not mandate a no-deal exit. (And, as we are talking blood in the gutters, I should add that the government’s Yellowhammer report on the price of no deal warned of “a rise in public disorder and community tensions”.)

I have read Cameron’s memoirs and can report that the overpromoted, overprivileged fool shows no understanding of how his actions brought party and country to this pass. Instead he chirrups that “our best days are ahead of us”. While we await Cameron’s New Jerusalem, the authorities must watch over all who run for public office. The Conservatives cut the police service that might protect them – to the point where it cannot solve 95% of burglaries and robberies. In the words of the committee on standards in public life, political intimidation is now “a threat to the very nature of representative democracy”. Good candidates won’t stand. The basic democratic condition that political ideas must be freely discussed in open debate has been undermined.

Facebook Twitter Pinterest David Cameron’s autobiography ‘shows no understanding of how his actions brought party and country to this pass’. Photograph: Will Oliver/EPA

In 2017, every single female MP who was active on Twitter reported online threats. I don’t mean to minimise the danger, but in the next election challengers could face graver threats. Jess Phillips, Anna Soubry, Luciana Berger, Yvette Cooper, Diane Abbott are targets because they are public figures, and because the thugs of left and right get their kicks from going for women. But at least these MPs have contacts with the local police, panic buttons at home and constituency offices with grills and double locks. A new Tory confronting a far-left crowd, a Labour or Lib Dem challenger confronting the far right, will not have their resources.

The committee offered 30 recommendations to protect the open society from its enemies in 2017. Next to nothing has been done. The committee and the Jo Cox Foundation tried to get a common code of conduct across all parties. Brandon Lewis for the Tories and Ian Lavery for Labour have yet to come to agreement. The committee wanted a new offence of intimidating parliamentary candidates and party campaigners, and for Twitter and Facebook to be forced to take legal responsibility for incitements to violence. The government wants to do it. But it has passed no new law because the referendum has condemned ministers and the rest of us to waste our lives debating Brexit to the exclusion of all that matters. To put it another way: we cannot be protected from the tensions caused by Brexit because of Brexit.

Over the past decade Cameron, May and Johnson created the conditions for perpetual grievance with a referendum whose result could never be honoured. Johnson has gone further and is fanning rage to inspire his base. His predecessors cut budgets for the law enforcement that might have deterred criminals and failed to adopt protective measures.

Let this be the Tories’ epitaph. Here lies the “party of law and order”. It died invoking mob rule. Here lies the “natural party of government”. It left an ungovernable Britain.

• Nick Cohen is an Observer columnist