There’s a case to be made that British democracy is not in dreadful trouble but working quite well. Or at least as well as can be expected in extreme circumstances. Even under the intense stress of the Brexit crisis, our spatchcock constitution is just about keeping it together.

Exhibit One. The prime minister exploits the prerogative powers to attempt a prolonged suspension of parliament that is designed to prevent MPs from invigilating the government at a critical time. The supreme court coolly steps up to this challenge to democracy, delivers a unanimous and impeccably reasoned judgment explaining why the prime minister acted unlawfully, and parliament is back in business the very next day.

Exhibit Two. The prime minister tries to manoeuvre towards a crash-out Brexit for which he has no mandate either from parliament or the people. The latent Commons majority opposed to that outcome mobilises to place a legislative block on him.

Exhibit Three. In the wake of the supreme court’s judgment, an utterly unrepentant prime minister comes to the Commons to accuse baying opposition MPs of sabotaging Brexit, leading to an unedifying exhibition of raw hatred on both sides. Even the ugly scenes on Wednesday night can be read as a sign that democracy is still performing one of its vital functions. As the old saying goes, better that people are shouting at each other in parliament than cracking heads in the streets.

About this last claim, I am no longer confident, because there are now some clear connections between incendiary political language and physical violence. In decades of writing about politics, a career that spans the revolt against the poll tax, the Iraq war and many other divisive convulsions, I have never known a time when so many parliamentarians live in fear that something terrible will happen to them or their loved ones.

The defining moment of Wednesday night was when the prime minister was confronted by Paula Sherriff, the Labour MP for Dewsbury. She told him that many MPs were receiving death threats that quoted his rhetoric accusing parliamentarians of “surrender” and “betrayal”. She reminded him that her colleague, Jo Cox, had been murdered during the referendum campaign by a far-right fanatic who in court shouted: “Death to traitors.”

We should note how casually these supposedly patriotic Brexiters seek to trash the institutions that made Britain great

The prime minister could have responded by saying they all had a responsibility to choose their words with care, especially when passions are fierce. He instead dismissed her: “I’ve never heard such humbug in all my life”, before later suggesting that the best way to honour the memory of Ms Cox was to “get Brexit done”.

A lot of outrage has followed, much of it deserved, some of it hypocritical. One of the better observations made by Jeremy Corbyn in recent days was that no party “has a monopoly of virtue”. All the parties bear some guilt for the poison flowing through Britain’s body politic. The Labour leader’s one-time promise to introduce “a kinder, gentler politics” rings very hollow with the Labour MPs who have been targeted with vicious abuse by some of his zealots. The referendum in Scotland in 2014 featured cybernats hurling the word quisling and other vituperative insults at Scots who opposed independence. The Liberal Democrats, who like to think of themselves as the saintly ones, are not above reproach. Their deputy leader, Ed Davey, had to apologise for expressing a desire to “decapitate that blond head” of Boris Johnson.

So it is not right to say that this toxicity all began on the day that Mr Johnson arrived at Number 10. It is right to point out that it has got worse since then. The prime minister, as the most senior representative of the political class, has the largest duty to set the tone, not to be the troll-in-chief. When he uses incendiary language, this not only inflames emotions, it conveys a sense of legitimacy to those extremists with violent impulses.

People around the prime minister have been spinning to the media warnings that there will be “a violent uprising” if parliament stands in the way of his desired form of Brexit. That sounds monstrously indistinguishable from blackmail. Amber Rudd is a former home secretary who, until very recently, sat in Mr Johnson’s cabinet. She does not say it lightly when she observes that “the sort of language… we’ve seen more and more coming out from Number 10 does incite violence”. Ms Sherriff was correct to remark: “We must moderate our language and it has to come from the prime minister first.”

I doubt she is going to be granted that wish. Incendiary phrase-making helped make Mr Johnson’s journalistic fame, powered the Leave campaign to victory in the referendum and eased his path to the premiership. Having been so far thwarted in his desire to trigger an election, there is, I suspect, an element of deliberately goading the opposition parties to bring on a no-confidence motion. Most importantly, Dominic Cummings, his chief strategist, is telling him that they will profit from deepening Britain’s division into two snarling camps. He sees a path to an election victory by sharpening the polarisation of the country on Leave/Remain lines and rousing a furious Brexit tribe behind them while the anti-Brexit forces are split. When the supremes pronounced Mr Johnson guilty of giving unlawful advice to the Queen, Remainers read this as a humiliating verdict that ought to lead to the prime minister’s resignation. Dominic “we are enjoying this” Cummings saw it as a supplement to the narrative being fed to Brexit voters that their wish to leave the EU is being thwarted by cunning continentals, pusillanimous MPs and out-of-touch judges. This is a divide-to-conquer politics that regards the amplification of hatred not with regret but as essential to electoral success.

They are planning a “people versus parliament” election and we will be very lucky if it does not turn into the nastiest contest in Britain’s modern history. It was not by accident that they alighted on the term “surrender act” to describe the legislative block on a no-deal Brexit. They started putting the phrase in the prime minister’s mouth after finding that it resonated with Leave voters in focus groups.

I am not priggish about political language. In a less charged atmosphere, “surrender act” would just be a topical example of a long history of negative labelling being used to try to discredit an idea. Both the left and the right do it, though the right tends to be more ruthlessly effective. The trouble is that we are in a super-heated context. “Surrender” is an incendiary phrase because it comes from the family of toxic branding that includes describing judges as “enemies of the people” and parliamentarians as “saboteurs” and “traitors”. We should note how casually these supposedly patriotic Brexiters seek to trash the institutions that made Britain great.

This confronts Mr Johnson’s opponents with a dilemma similar to that which has faced the opposition to Donald Trump. Call out fire-raising language and you risk supplying it with additional oxygen. Each time that the opposition complains about “surrender act”, the phrase receives additional airtime and Mr Cummings punches the ceiling with delight. We know that it was with deliberate provocation that the Leave campaign painted its battle bus with that infamous slogan about the EU and NHS spending. The more their opponents attacked the claim as mendacious, the more the national conversation was dominated by arguments about the cost of EU membership rather than its benefits.

Some of Mr Johnson’s opponents will seek to pay him back in the same corrosive coin, responding to his incendiary language with their own. Overheated Remainers fell into that trap when they described the prorogation as a “fascist coup”. A different way of deflating demagoguery was demonstrated by Clement Attlee during the 1945 election. Winston Churchill made the outrageous claim that a Labour government would introduce “some form of Gestapo” to impose Nazi-like totalitarianism on Britain. Attlee came back with a pitch-perfect response: “When I listened to the prime minister’s speech last night, in which he gave such a travesty of the policy of the Labour party, I realised at once what was his objective. He wanted the electors to understand how great was the difference between Winston Churchill, the great leader in war of a united nation, and Mr Churchill, the party leader of the Conservatives.”

Michelle Obama said: “When they go low, we go high.” That’s my preference, but I say that knowing that Boris Johnson and Dominic Cummings will regard it as a motto for losers.

• Andrew Rawnsley is Chief Political Commentator of the Observer