Keir Starmer has neither a heartland nor a stronghold. That is the picture likely to emerge once all the votes in this week’s local and devolved elections have been counted.

Council seats in Labour’s traditional northern-English working-class base will fall to Reform UK. Parts of inner London, where the electoral map has been red for decades, will go Green.

The Scottish National party will still be the biggest party at Holyrood, thwarting Labour’s hopes of ending its banishment from power there. If opinion polls are not mistaken and Plaid Cymru becomes the largest party in the Senedd, it will bring an epic run of Labour dominance in Welsh politics to an end. The party hasn’t been in opposition since the formation of a devolved assembly in 1999. And that record reflects a cultural primacy dating back a lot further.

Northern Ireland and Scotland already have first ministers whose parties are opposed to union with England. Wales will join that number if Plaid’s nationalist leader, Rhun ap Iorwerth, forms the next government at Cardiff Bay.

That wouldn’t sound a death knell for the UK, but it would be a symbolic fracture. Downing Street will look ridiculous trying to pretend that such results are an expression of normal midterm turbulence. Even in the best-case scenarios available to Labour from current polling, Starmer will look like the caretaker leader of a party that struggles to say who its core voters are or where they might live. (Manchester, maybe.)

The Conservatives are not faring much better. Their electoral base has been partitioned along a Brexit faultline. Reform appeals to angry, disillusioned leave voters. The Liberal Democrats are consolidating their hold over the remainer belt in what used to be true-blue Tory suburbs and shires. The two-party duopoly that defined British political competition in the 20th century has broken down everywhere, except the Palace of Westminster. Labour and the Tories are still the big beasts in the chamber where laws are made and ministers are held to account, but that constitutional primacy looks like a relic from another era.

Zack Polanski, leader of the Greens, is not an MP. Reform’s Nigel Farage notionally represents Clacton in the Commons, but his time and energy are mostly spent elsewhere.

In that respect, England is following trends that are well established in the devolved nations. Labour’s first “red wall” to fall, years before that metaphor was applied to Boris Johnson’s post-Brexit march through the Midlands and northern England, was in Scotland. It was demolished by the SNP.

Some of that terrain was recovered in Starmer’s landslide 2024 general election win. That only makes it more galling for Scottish Labour to be facing another term of opposition in Edinburgh; to see the SNP, weighed down by failures and scandals accrued over many years in office, somehow defy political gravity.

It helps the SNP to have a bedrock of supporters for whom independence is a cause to trumps all others. It helps even more that the pro-union vote is fragmented and that the Westminster government is hated. Anas Sarwar, Labour’s leader in Scotland, has repudiated Starmer but the national brand is still an albatross round the Scottish party’s neck.

Eluned Morgan, Wales’s first minister, has the same problem compounded by double incumbency. Under Tory prime ministers, blame for whatever went wrong in Wales could be deflected on to wicked Tory rule in Westminster. Starmer’s arrival in Downing Street withdrew that device. Change was promised and not delivered. For Welsh voters of a leftish disposition who are fed up waiting, Plaid Cymru offers a multi-use electoral tool: try something new; punish Labour; prolong Tory exile; block the forces of Faragism.

That confluence of motives doesn’t amount to a surge in demand for independence and Plaid’s leadership know it. The prospect of ending union with England is buried in the manifesto as part of an “ongoing national conversation about the options” on the future, with a vague commitment to a white paper that might one day raise the constitutional question that used to be the party’s defining purpose.

But a Welsh nationalist government could still effect the kind of systemic drift that has made Scottish politics feel increasingly remote from the rest of the UK, without drastic alteration to the constitution. Plaid, like the SNP, will be able to govern from a stance of perpetual opposition. They can frame every UK-wide debate as a question of who can be trusted to stand up for Wales without conflicting allegiance.

The lesson from Scotland is that opposition leaders, operating in the shadow of English parent parties, find it very hard to wrestle control of the agenda back once it is set in those terms.

The challenge could be even greater if Farage becomes the standard bearer for unionism. His party is poised to come second in Wales and might do the same in Scotland.

Reform’s Scottish and Welsh supporters mostly care about the same issues that have driven the party’s growth in England – immigration, economic insecurity, general antipathy towards Westminster politics. Constitutional structures are not much on their radar. That won’t necessarily prevent the union question getting sucked into a feedback loop of polarisation and mutual radicalisation. Not if Faragism sets the tone of resistance to Welsh and Scottish independence movements.

Reform’s anglocentric, Brexit-coded, racially inflected mode of British nationalism is a pungent brew that could give undecided, moderate voters in the devolved nations a taste for rupture from England. A sharpening of pro-independence demands will then animate the resentful streak in English nationalism that sees the existing constitutional setup as a scam, siphoning resources from an enterprising motherland to ungrateful Celtic dependants.

There is a precedent for these dynamics in the breakup of another asymmetrically weighted, multinational state. The collapse of the USSR started with secession demands on the periphery, but it became inevitable once Russia itself – rallying to Soviet Russia’s ambitious national president, Boris Yeltsin – moved for dissolution of the union.

The comparison is flawed in countless ways. The UK is not an authoritarian, one-party communist regime with hardline generals plotting to resist liberal reform. Our traditions of pluralism and democracy have deep roots. The economy is troubled but not in anything like the condition of abject failure that made the Soviet system unviable. Any likeness in the two cases is a matter of historical rhyme, not analytical rigour.

Continuing for a moment in that spirit, it is possible to glimpse in Starmer a hint of Mikhail Gorbachev – the reforming apparatchik who underestimated the scale of the challenge before him, lost control of centrifugal forces and ended up stranded as leader of a country that didn’t exist any more.

Meanwhile, back in the realm of evidence, the votes have not even been cast in this Thursday’s ballots. The vagaries of the different electoral systems in play make a wide spectrum of outcomes available. But a safe prediction is that the map of British politics, shaded by party representation, will be a more Technicolor mosaic after these polls than it is now. There will still be patches of red, but it will be hard to make a coherent pattern of them. Britain will still have a Labour government with a huge majority in parliament. But Starmer will lead a party with no place to call home.

• Rafael Behr is a Guardian columnist