More people in England have already forgotten the name of the Scottish finance minister who resigned last week than ever knew it before the scandal broke. Derek Mackay was hardly a celebrity in Scotland, but his disgrace made news there for more than a few hours. Mackay had compulsively messaged a 16-year-old boy on social media, calling him “cute” and offering him trips out. The behaviour was “foolish”, he said. Opponents called it predatory.

Questions followed about the Scottish National party’s handling of the matter and how its leader, Nicola Sturgeon, should patch the hole in her cabinet, but England had tuned out by then. Scandal is union-wide; aftermath is devolved. This isn’t new. People have limited bandwidth for politics, and the SNP doesn’t govern England. Holyrood is a niche interest even for Westminster nerds. But the longer this segregation of political cultures goes on, the harder it gets to argue for a United Kingdom.

Sturgeon is counting on Brexit to drive divergence to the point where independence feels irresistible. Two-thirds of Scots wanted to stay in the EU, and their ejection from it on Boris Johnson’s terms reinforces old resentment of rule by remote English Tories. Sturgeon hopes fresh anger will keep her party buoyant through next year’s Scottish parliament elections despite a heavy freight of incumbency. The SNP has been running Scotland for nearly 13 years – time enough to write a catalogue of troubles, of which the Mackay saga is just an appendix. Later this year Alex Salmond, Sturgeon’s predecessor, goes on trial over allegations of sexual assault and attempted rape.

So far the SNP has relied on the inadequacy of the opposition to weather storms, and there is no sign of that shelter being withdrawn. If you had forgotten his name, Richard Leonard is Labour’s Scottish leader but not the answer to any other questions. His party has fluffed the issue of a second independence referendum, and not just because Jeremy Corbyn was eyeing deals with the SNP to build a Commons majority. The radical English left already had a soft spot for Scottish nationalism. Corbyn likes causes that parade as liberation struggles, and avoids positions that involve defence of the British state in its established form. A similar impulse persuades Scottish Greens to support independence.

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Non-aligned unionists miss Ruth Davidson’s voice in the debate. The former Tory leader in Edinburgh was an adept reader of Scottish politics, and proved it by quitting rather than waste her skills defending Johnson.

The SNP benefits also from a shift in attitudes since Brexit. Polls show discernible movement towards independence, driven by remainers who voted no in 2014. There is demographic momentum too. Younger voters like independence. But Sturgeon’s strongest card is the total capture of Westminster by Tories, with the prime minister’s anglocentric swagger providing bonus repulsion of Scottish voters.

Johnson’s administration also makes it hard for non-Tories in England to plead the unionist cause. In theory, ancient bonds are precious under any government, and as worthy of defence outside the EU as in it. Tories can be ousted, while cutting the UK’s constitutional cord is forever. Anti-Brexit arguments should also translate into rebuttal of the case for Scottish independence. In both scenarios it is unwise, impractical and expensive to repaint borders that progress had erased. But fewer remainers have the heart to push that line when the union whose virtues they must extol has Boris, Brexiteer Supreme, as its champion.

Play Video 1:24 Nicola Sturgeon: independence best for Scotland post-Brexit - video

Facing at least five years of Tory government, many English voters might envy the Scots their referendum escape raft and their dreams of sailing it back to Europe. Sturgeon was in Brussels on Monday advertising her readiness for that journey. Even if it looks like a long shot, plenty of demoralised remainers will root for her if the alternative is cultural capitulation to Jacob Rees-Mogg and Nigel Farage.

While moderate unionist voices are muted, the noisiest opponents of Scottish separation are tin-eared Tories who can’t praise the heritage of Great Britain without sounding as if they mostly mean England. Even if Johnson aims for an emollient tone he will be quickly baited into confrontation with the SNP over demands for a referendum. The Tory leader has only two settings: frivolous and contemptuous. He visibly struggles not to despise those whom he cannot entertain.

Incumbency will also weigh on the Tories. Brexit promises will be pulped in the negotiating mill, and foreigners will be blamed. Johnson will sound increasingly like an English nationalist. The prime minister thinks his permissive social attitudes and free-trader rhetoric make him a liberal. It is true, up to a point. Yet that point is reached whenever expediency requires a tilt into chauvinism and xenophobia. Johnson embodies a common delusion that the English are inherently resistant to nationalism, seen as a vulgar condition afflicting less pragmatic nations.

Scotland’s independence campaigners have a similar trick, insisting theirs is the good kind of nationalism – “civic”, “progressive” – not like the mean, atavistic strain that sows conflict and division. They too have a point. Sturgeon is no despot, and the SNP has egalitarian intent. But the test is not the ambition with which nationalism starts, it is the turn taken when utopia is not reached. The beginning is always democracy, freedom and romance. The end is always disappointment and a hunt for traitors. Nationalists never own their mistakes because there is always some remnant of the old regime or a foreign neighbour to scapegoat.

Whether the bad guys are in Brussels or Westminster, the model has the same flaw. The idea that the collective destiny of a nation can be truly expressed only by the victory of one party or one leader is a well-trodden historical dead end. Nationalism rejects the moral validity of rival programmes. It demotes tolerance and, when short of fuel, taps the rage of fanatics. It is poisonous to liberal democracy, although democratic principle demands it be given space on a ballot paper. Once there, it stands out in bold colours against politicians who deal in grey shades of complexity.

Those of us who wanted to stay in the EU, and also want to keep the union of England and Scotland, lack compelling stories to rival the myths that drive separatism. It is a grimly familiar sensation – watching from the sidelines, muttering boring old facts while the fate of our country is settled by better storytellers. History is meant to be our vaccine against nationalism, but it doesn’t work when we let nationalists rewrite the past in order to spell out the future in their own words.

• Rafael Behr is a Guardian columnist