When a commentator first referred to the ‘Ulsterisation’ of Scottish politics, he was jumped on by people keen to take offence at the parallel.

But whilst Scotland does not stand on the brink of civil war – and the coiner of the phrase did not claim otherwise – there is no disputing that Northern Ireland offers an insight into what politics looks like when it gets polarised around the constitutional question.

It is not hard to see parallels with the Ulster Unionists, for example, when the Scottish Conservatives put their short-term electoral interests ahead of what’s best for the UK by parroting the SNP’s central election pledge (‘Vote for us to get another referendum’) on their own literature, to such an extent that it is undermining their efforts to criticise the Nationalists for focusing on independence.

But by far the worst artefact of capital-U Unionist politics to have been dragged into the Scottish debate is the idea of a ‘Unionist unity’ pact.

This is one half of the grand electoral wheeze which was originally supposed to be All for Unity’s justification for existing. The idea was that the three major pro-UK parties would divide up the first past the post constituencies between them and then abandon the lists to A4U, thereby maximising the overall number of anti-separatist MSPs.

Both halves of this idea are deeply, indeed fatally, flawed. But a proper examination of the problems with the list half – which involves tangling with the complexities of the D’Hondt system and examining a lot of assumptions that dramatically change what you get from the data – will have to wait for another time.

The problems with an FPTP pact, however, are obvious. And with angry A4U supporters continuing to damn the big parties for failing to conclude one, it is worth setting them out again.

First, on the practical side, there is just scant evidence that such a pact would be anything other than a boon for the nationalists. Crude calculations based on adding up the combined vote for pro-UK parties in a seat fundamentally misjudge the electorate.

Time and again, Labour and Liberal Democrat voters have proven deeply resistant to transferring to the Conservatives – sometimes to such an extent that the Tories can’t close a 20-vote shortfall with the SNP in a multi-round local by-election. When Labour’s vote collapsed again at the 2019 general election, it was the SNP who profited. Even Conservative voters stubbornly refused to help Jo Swinson hold East Dunbartonshire, and she’d only have needed 150 of them to do it.

The blunt truth is that the total combined vote of the pro-UK parties is not a perfectly fungible ‘Unionist vote’. The simplest way to think about it is as a Venn diagram: the potential vote of a Unionist pact is not the total area of the three circles, but only that of the overlapping sections.

So the three main parties combined would be much less than the sum of their parts. And that’s even assuming they could negotiate a three-way division of seats without it devolving into a complete circus, the way those between the Liberals and the SDP did in the 1980s.

(None of this is to say that committed Unionist voters shouldn’t vote tactically if they wish to. But a lot of voters for pro-UK parties are not ‘committed Unionist voters’, and won’t.)

But not only would such a pact be a serious short-term misstep, the long-term strategic consequences would be even more severe.

Any deal stitched together between parties with such different philosophies would be based purely on their top-line constitutional position. They would, therefore, find it very difficult to talk about anything else. At a stroke, the ability of any party involved to appeal to swing voters for whom the constitution is not top priority would disappear.

Likewise, once you start throwing anything and everything overboard for the sake of scrounging a few extra seats, it isn’t long before you start eyeing up those weighty commitments to broader British politics. If you’re not going to be a liberal, what is the point of being a Liberal Democrat? If you’re no longer putting forward a distinctly conservative vision for Scotland, why trouble yourself with a formal link to the Conservative Party?

Some in A4U have already embraced this logic: Linda Holt, one of their candidates, recently called for the Tories to slough off responsibility for ‘defending mothership UK-wide policies’ in exchange for more freedom to ‘fight wholeheartedly for Scottish interests’. And they’re not the first. When Murdo Fraser proposed a separate Scottish Conservative party, one of his supporters wrote that it would have been ‘a Scottish party taking the London whip at its own discretion; not a London party cracking the whip in Scotland without knowledge or consideration’.

We only need to look at the unhappy state of Northern Irish unionism to see where the model of a big-tent Unionist party with no ideological or institutional bonds to the parties of government ends up. Far from strengthening the Union, it has seen politics too often reduced to a clash between two nationalist parties, one of which sports Union Jack branding, but with all the ideological content of the Old Firm.

In the long term, saving the UK means saving the British nation that underpins it, and preserving Scotland’s British character has to involve preserving, as best we can, British politics. Doing so helps shift the focus of politics away from the constitution, ensures voters have the breadth of choice they deserve, and maintains the paths by which Scottish figures can rise to the top of national politics.

The alternative is to steer Unionism into the Ulster cul-de-sac, all for the sake of scrounging a few jobs. Thank goodness that all the major parties have, so far, known better.