As a rule in politics, a successful TV appearance should not result in an apology to nuns. You arrive in the studio armed with a policy, tested in focus groups and proven popular, insisting that state schools hire only qualified teachers. There is no “party line to take” on cloistered orders. Hours later, newspapers say you have damaged Labour’s election chances in Catholic communities. Social media is chronicling every stage of your journey from original “slur” to contrition.

So what did Tristram Hunt do on BBC’s Question Time last week to provoke this reaction? “These were all nuns,” the shadow education secretary said. Written down, it looks disappointingly lacking in anti-clerical venom.

Even italics can’t convey the heresy, because it wasn’t the words that made the story. There was an exchange between Hunt and Cristina Odone, a journalist and commentator on Catholic matters, who cited her own happy education as refutation of Labour’s argument. Hunt wanted to make a point questioning the relevance of a rarefied religious schooling to his wider policy. But in the heat of the moment he went all high-pitched, knitted his brow in condescending perplexity and dropped the n-bomb. Soon the virtual hills were alive with the sound of synthetic partisan outrage.

So the charge is body language insufficiently reverential to the idea of devout Christian womanhood. The aggravating circumstances are possession of a posh voice, a private education and ministerial ambition.

Not appearing to slag off nuns on telly is something you either learn to do or lose the game

The standard Westminster view is that these incidents are part of the rough and tumble of politics. Not appearing to slag off nuns on telly is something you either learn to do or lose the game; no sense complaining. All parties rely on their enemies’ semantic accidents for tactical advantage. When welfare minister Lord Freud was recorded wondering aloud whether disabled people might get on better in the labour market if paid less than the minimum wage, Ed Miliband raised it in parliament and secured an apology.

Freud’s defence was that he was speculating in response to a question, not stating an opinion, let alone a policy. The riposte is that by indulging the premise of the question, the minister showed latent contempt for disabled people. The veil of compassion was said to have fallen away, exposing the government’s nasty core.

There was no excuse of “words taken out of context” because the conversation was on tape. The transcript didn’t exonerate Freud but it did confirm that he had been busking freestyle around the topic. He hadn’t really thought the matter through, and said so honestly. A shrewder politician would have known that the only safe way to express not having thought something through is silence.

This is the difference between public political discourse and other kinds of conversation. In most human interaction there is a safety net woven of mutual effort at comprehension and a right to retract. I want to understand you as you want to be understood. You let me correct myself if I was unclear. Not in politics, where every statement runs a gauntlet of malicious misinterpretation.

This trend is further advanced across the Atlantic. “You leave the charitable realm where people cut you some slack … and accept that you didn’t quite mean what you said,” writes Michael Ignatieff in Fire and Ashes, an account of his failed career change from academic to leader of Canada’s Liberal party. “You enter a world of lunatic literal-mindedness where only the words that come out of your mouth actually count.”

Meticulous care with words can’t save you if a stray sharpness in tone is taken to reveal an inner metropolitan sneer

Actually, it is worse than that. Meticulous care with words can’t save you if a stray curl of the lip or sharpness in tone is taken to reveal an inner metropolitan sneer. Al Gore sabotaged his chances of becoming president of the US by rolling his eyes at George W Bush’s inadequate answers in a TV debate. The commander in chief is meant to be smart, but not smug about it.

We are heading the same way in Britain. The cult of “message discipline” is eating itself – a vicious circle of leaders imposing rigid scripts on their parties to avoid stories of disunity, forcing media to extrapolate big divisions from small scriptural deviation, making leaders ever more paranoid. The rewards available for saying something original are tiny when set against the risk of ridicule and pillory. The effective candidate is one who can improvise fluently around a theme without saying anything new, painting a void in the colourful inflexions of ordinary speech; vernacular evasion. David Cameron is good at this; Miliband is not. Most MPs sound robotic with the effort.

It is a climate in which the artful maverick flourishes. Nigel Farage’s political gift is to speak as if without internal censorship. His views appear to pass from impulse to mouth without being security-vetted by a press office in the brain. It is how most people speak most of the time, but not most politicians. So Ukip expresses anger at the way the complexion of the country has been changed by immigration, and claims to have unique authority on the subject. It pretends to be defying a conspiracy of silence, although there was never any silence. There was a lot of talk to which hardly anyone was listening.

Obsessive-compulsive message hygiene by the big parties encourages a puritanical media hunt for specks of inconsistency and deviation. Then mainstream debate becomes sterile and the fringe more fertile. We cherish free speech but have become tyrannically unforgiving of the misspoken word. There is no benefit of the doubt, no concession for an honest motive poorly executed. It is meant as a defence against dishonesty and hypocrisy. But when the presumption takes hold that all players are hiding their true selves, politics becomes a parlour game where the aim is to trip your opponent so their mask slips.

Then, when nationalist demagogues arrive on the scene with an agenda that history has shown to be divisive and sinister yet also seductive when portrayed as the antidote to a corrupt establishment, what happens? The mainstream parties and commentators say: “It is a facade behind which lurks something ugly. These people blow a dog-whistle. They say one thing but mean another.” And the response, dangerously plausible, comes back: “No, it is you who wear the masks and speak in codes. We are the plain speakers and the unmaskers.”