If Boris Johnson is capable of shame, then he should surely now be feeling it. He has brought this country to the point where an election is genuinely to be feared, no matter who wins, because of the violence that may follow; where MPs facing death threats will need all the courage they can muster just to go out campaigning, and decent voters will switch off from the spite and hysteria surrounding them.

It should go without saying that Johnson’s dismissive response to the Labour MP Paula Sherriff, when she challenged his deliberately provocative talk of surrender and betrayal over Brexit, was awful. Having listened in the past to Sherriff matter-of-factly describe the threats she has faced from the far right, and the depressingly elaborate precautions so many of her colleagues must take just to go about their everyday life, I too feel rage bubbling up at the idea that this was somehow faux concern. But what if rage is what they want, those determined to turn this next election into some kind of political hunger games? For only when we are goaded beyond reason, red mist descending at the slightest provocation, can they get the reaction their divide-and-conquer strategy needs.

All of us have a choice over whether to pour more petrol on the flames, although that responsibility clearly weighs far more heavily on the prime minister than on anyone else

As Jo Cox’s widower, Brendan, gently pointed out this morning, political language is now sliding out of control on both sides. Emotions are running so high that we are all slightly losing the plot. The word “fascist” is cheapened by overuse on the left, along with talk of coups and unelected dictators and witch-hunts. There wasn’t much milk of human kindness on show in Brighton either, where Labour’s party conference began with an attempt to abolish Tom Watson and ended in reports of a grim little gathering outside the official conference fringe to protest against expulsions on the grounds of antisemitism. If it’s dangerous for Brexiters to toss around words such as traitor and surrender, it’s hardly edifying for the suspended MP Chris Williamson to compare party officials charged with rooting out this racism to “the Stasi”.

To claim that hate-filled leftwing rhetoric doesn’t really count, because the far left hasn’t murdered anybody, meanwhile, is to overlook not only British Jews’ real fear of violence but the way one form of extreme language provokes another, driving everyone to the edge of things that should not be said. All of us have a choice over whether to pour more petrol on the flames, although that responsibility clearly weighs far more heavily on the prime minister than on anyone else.

For all the understandable emotion in parliament, this may not prove much of a turning point for voters. The first rule of politics is that everything matters, but nothing matters as much as it would if more people were actually paying attention. Most voters are too busy cooking supper, supervising the kids’ homework or watching box sets to spend their nights glued to BBC Parliament, and millions are now so sick of Brexit that they are shutting out political news altogether. Among those interested enough to catch the headlines, remainers who fear the country is heading for dark places will have had their worst fears confirmed, while hard Brexiters who hate having their language policed won’t see what the fuss is about, any more than they did whenever Johnson casually tossed something incendiary into a newspaper column. But if it doesn’t shift entrenched political allegiances, that doesn’t mean it doesn’t matter.

Should Johnson ever bring back the deal he says he wants, it’s now infinitely harder for Labour MPs who instinctively wanted to support it to vote with him. The pressure on moderate Tories still in government to quit rises hourly, and tensions within No 10 over Dominic Cummings’ aggressive tactics are bubbling to the surface. Downing Street sources – and I think we can all guess which Downing Street sources – came out swinging, comparing Johnson’s use of the phrase “surrender act” to describe the legislation aimed at preventing no deal with that notoriously inaccurate referendum pledge of £350m a week for the NHS. All controversy is good controversy, we’re told. Liberal outrage only bolsters the idea of a showdown between parliament and people.

Well, maybe. Perhaps Johnson really did meticulously plan what he’d say to someone like Sherriff, rather than instinctively lashing out with only the vaguest idea of her history. But as the supreme court ruling suggests, the sledgehammer tactics that worked for Cummings in a referendum don’t always translate so well to Westminster. Much now depends on how well the man who once boasted of never having joined the Conservative party really understands the mood inside it, turning this week’s party conference in Manchester into a crucial litmus test.

The night after the supreme court ruling, the former Liberal Democrat leader Tim Farron tweeted that he was seeing lifelong Tory voters switching on the doorsteps, reeling from what he described as a sense that “the Tory party has become uncivilised”. And if that’s exactly what a mischievous Liberal Democrat would say, you don’t have to scratch the surface of grassroots conservatism very hard to find a quiet unease even among activists who have been loyally knocking on doors for decades.

They may have gone underground since David Cameron resigned, but there are still legions of instinctively Tory voters out there in what are no longer wholly safe Tory seats who value good manners, steady leadership and a quiet life. Many still hope that once Brexit is done, their party can return to what they regard as its senses. But it’s beginning to dawn on some that if that’s what they want, then they need to fight for it. If moderate Tories don’t find their voice in Manchester this week, they may wake up and find it’s too late.

• Gaby Hinsliff is a Guardian columnist