Shortly before the second general election of 1974, the late John P Mackintosh attempted to explain the rise of the Scottish National Party to a predominantly left-wing (and English) audience in an essay for the New Statesman.

All the devolutionary schemes then being bandied around, he judged, missed the point, for the idea that SNP fires were “fuelled” by the desire for a “subordinate assembly” was clearly a nonsense.

Rather the party had skilfully “manoeuvred” Scotland’s Unionists into having to offer some degree of devolution or risk appearing “totally insensitive to the situation in Scotland”. “They have forced the other parties,” wrote Mackintosh, “to fight on ground chosen by the SNP.”

And, of course, anything the Unionist parties suggested would “always be inadequate, be it more regional economic advantages or assemblies or a percentage of oil revenues”. Unless they succeeded in building up an alternative proposition, the SNP – which wanted nothing less than independence – would eventually get what it wanted.

A few years later, Mackintosh died aged only 48, and re-reading his work today it’s clear what a loss he was. Later this week an event at the Scottish Parliament will mark the anniversary of his death with a debate grandiloquently entitled “Where now for democracy?”

Yet ruminating on the legacy of Mackintosh isn’t an entirely happy affair, for his twin constitutional passions – Scottish devolution and European unity – are now, in different ways, once mainstream propositions that find themselves out of favour.

Take the latest tribulations over Brexit. The only party unequivocally in favour of reversing the decision taken by a majority in June 2016 are the Liberal Democrats; everyone else is desperately trying to square circles in order to unify parties divided between Leave and Remain. And without clear convictions, there’s little political courage.

Thus, the Conservative Party appears terrified of a hard Brexit even though that is the logical conclusion of its actions, while Labour, despite the usual noises about Jeremy Corbyn “evolving” his position, doesn’t meaningfully depart from the Government’s muddled position. The SNP, meanwhile, sits on the fence regarding a second referendum and fights for membership of the Single Market while remaining conspicuously silent on any future application for re-admittance to the EU.

Mackintosh wouldn’t have been surprised by the posturing surrounding Clause 11 of the EU Withdrawal Bill, still ongoing despite a significant climb down by Westminster. Even when London signalled its retreat, the Scottish Government kept on fighting, filling newspapers with righteous indignation; the entire devolution settlement, they stormed, was on the brink of destruction.

The SNP, however, make unconvincing defenders of a Scottish Parliament they only came around to supporting when the 1997 referendum (and a Yes-Yes vote) was an inevitability. Nor have ministers in London covered themselves in glory. Theresa May’s warm words back in 2016 about working in concert with the devolved administrations to deliver Brexit turned out to be just that, warm words.

Devolution within a quasi-federal framework can only work if both levels of government actually want it to, and therein lies the central tension identified by Mackintosh back in 1974. The SNP is not a devolutionist party, and while Labour and the Conservatives are not as actively anti-devolution as some Nationalists claim, they certainly regard legislatures in Edinburgh, Cardiff and Belfast with a degree of frustration mingled with incomprehension.

Sir Nick Macpherson, formerly permanent secretary at the Treasury, recently wondered aloud on Twitter how long the UK Government would continue to “prioritise its obsession” with mainland Europe over the historic union with Scotland, Wales and Northern Ireland. He added an ironic hashtag, #ageofenlightenment, for of course the UK Government’s approach is anything but enlightened.

The front page of a prominent Sunday newspaper underlines the shift in tone. Where previously ministers have been conciliatory, Cabinet Office minister David Lidington now warns the Welsh and Scottish governments could cause the UK to leave the EU “as a country split and an economy disjointed, struggling to make our way in a new world outside the EU”.

Now the SNP is many things but the idea it’s poised to pass different food safety and chemical standard laws purely to annoy Westminster lacks credibility. Similarly, Nationalist arguments that every single power once shared with Brussels ought to end up in Edinburgh is an interesting interpretation of the 1998 Scotland Act, but not a wholly convincing one.

Nor is it likely a different approach from the UK Government would have made much difference. The SNP see in Brexit an opportunity, an opportunity yet to pay electoral dividends to be sure, but an opportunity nevertheless. This means they don’t really want to make nice with wicked Tories in London.

Even so, Mackintosh would surely despair at the state of Unionism in 2018. He spoke of most Scots’ “dual identity”, simultaneously Scottish and British, yet the latter of those now consists of Darkest Hour-type nostalgia at the movies and quixotic campaigns to relaunch the Royal Yacht Britannia. Scotland in Union, never a terribly credible outfit, is now split and begging for cash.

“Pooling and sharing”, Gordon Brown’s contribution to the Unionist argument before the 2014 referendum, remains empirically true (the Barnett Formula, much like the NHS, has not been swept away) but a hard sell amid the likely economic consequences of Brexit. Unionists continue to rely on the self-evident weaknesses in their opponents’ arguments rather than any defence of the status quo that truly tugs at the heart strings.

One of John P Mackintosh’s most famous quotes is emblazoned within the walls of Holyrood. “People in Scotland want a degree of government for themselves,” he declared two years before he died, “it is not beyond the wit of man to devise the institutions to meet these demands.” That was certainly true in 1976, today I’m not so sure.