The next Tory leader will face a vital decision. Let’s call it the ‘rivers of blood’ test | Rafael Behr

The last four times the Tories chose their leader, the rest of the UK was obliged to pay attention because the winner of the process would automatically become prime minister. The stakes this year are much lower. The prize is stewardship of a battered party reduced to a 121-seat rump in the Commons, exiled far from government.

The next leader’s first challenge will be adapting to a spectacular fall from relevance. There are nuances to be teased out of July’s election results, but the main message was an instruction to Tories from voters to shut up and leave them alone.

The contest to replace Rishi Sunak can be important without being compelling. Leader of the opposition is always an office of consequence. Those who don’t look credible as potential prime ministers end up empowering others with their failure, strengthening incumbent governments or creating space for rival opposition forces to emerge.

There is no way to narrate the long-term significance of David Cameron’s general election victory of May 2015, for instance, without weighing also the impact of Jeremy Corbyn becoming Labour leader four months later.

Most of opposition is internal party management. The memorable choices tend to be adjudications of doctrinal and rhetorical boundary disputes: who speaks for the mainstream; who is on the fringe; what kind of language is beyond the pale.

In that genre, one of the most important decisions taken by a Conservative opposition leader – newly resonant in the aftermath of far-right riots across England – is Ted Heath sacking Enoch Powell from his shadow cabinet in 1968.

The shadow defence secretary’s offence was his speech decrying national ruin by way of mass immigration, best known for the histrionic peroration conjuring an image of “the River Tiber foaming with much blood”.

Heath declared the speech “racialist in tone and liable to exacerbate racial tensions”. For a generation of Conservative politicians, that verdict marked the unofficial frontier where reasonable demands for robust immigration control crossed into dangerous rabble-rousing. The “rivers of blood” test was imprecise yet enforceable by collective cultural intuition.

Tory politicians didn’t stop fretting about migrant numbers, but they generally did so from what, in hindsight, looks like the civil side of the Tiber. The demand for stricter rules came prefaced with paragraphs celebrating Britain’s noble record of welcoming refugees and cherishing the contribution that foreign-born citizens have made to the country.

It is easy to dismiss that as cynical cant when the core message is that good immigrants are the ones who came in the past, while the latest arrivals are the wrong sort, making bogus claims for asylum and jumping the queue for jobs and services.

But the enduring obligation for Tory politicians to stay on nodding terms with the liberal case for immigration, to deploy it even if only as window dressing in their speeches, testifies to the thoroughness of Powell’s defeat.

His core assertion was that fruitful integration of newcomers was an illusion and Britain was existentially imperilled by the prospect of foreign-born citizens outbreeding the indigenous white population.

Faced with that horror, he argued that laws protecting minorities from discrimination were “exactly and diametrically wrong”. The real victims of prejudice and discrimination were not immigrants but “those among whom they have come and are still coming”.

The point of no return, the date of inflexion when Britain would be irretrievably submerged under the incoming demographic tide, was projected to be 1985. And yet, 39 years after that deadline, Britain is still Britain. The far right can cause a few nights of mayhem on English streets in 2024, but the nation is no closer to the civil war that Elon Musk gleefully forecasts on X now than it was to rivers of blood 56 years ago.

The older the speech gets, the more hysterical and irrational its assertions look. The richer it then becomes as a refutation of arguments that a new generation thinks they are discovering for the first time.

The complaint about “ two-tier policing ”, punitively tilted against white people; the lament that “woke elites” are in denial about the refusal of migrant communities to integrate; the confidence that, at a certain numerical threshold, civil insurrection by dispossessed indigenous folk becomes inevitable; the affected recoil from violence that is really a frisson of relish at the prospect – it was all prefigured, wrongly, by Powell.

A lesson for the next Conservative leader is that the line drawn by Heath has not moved, even if the party’s centre of gravity has shifted in relation to it. Over many years, the mainstream British right crept down to the banks of the Tiber. Nigel Farage led the way, paddling in the shallows at first, testing the tide of liberal opprobrium and finding he could comfortably swim against it. He swam ever further from the bank, making the case for Brexit as a remedy to foreign inundation . Then came the dread of small boat crossings, depicted as an “invasion” and a “swarm” by Conservative MPs.

Now those metaphors are taken literally by thugs who think they are repelling the alien horde with barrages of bricks and bottles. The demand to “stop the boats” and “take our country back” is backed with threats of arson and assaults on police officers. Now see the Tory leadership candidates thrash and flail back to the riverbank, hauling themselves from the bloody foam, spluttering about “thugs” and “extremists”.

Related: We keep hearing about ‘legitimate concerns’ over immigration. The truth is, there are none | Maya Goodfellow

They waffle vaguely about root causes of social malaise in terms calculated to sound disapproving of rioting without offending anyone who might sympathise with the rioters’ declared grudges. They pivot, without agility or logic, to the demand that an entirely different set of protesters, pro-Palestinian marchers who didn’t riot several weeks ago, really ought to have been arrested.

It is lucky for whoever becomes the next Conservative leader that no one is listening. The contest doesn’t conclude until November, which leaves time for one of the contenders to say something meaningful about modern Britain and why much of it rejected Tory rule with a shudder of revulsion. The riots were a test, but not of whether the party has a workable immigration policy or knows how to condemn a racist mob. That sets the bar pretty low.

The challenge in the coming months and years will be invigilating the line where responsible Conservatism, craving rhetorical potency, is tempted to raid the idiom of far-right demagoguery. Setting the parameters of civil dissent against an incumbent government is one of the constitutional duties of an opposition leader.

The next Tory leader will hopefully conclude that moderate language around migration is ultimately in the party’s electoral interest. But for the cause of British social cohesion the winner of the contest must also make it their mission to march the party back from the brink of the Tiber.

• Rafael Behr is a Guardian columnist

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