Is unionism in terminal decline? Those fresh from the weekend’s All Under One Banner march in Edinburgh will feel certain that it is. But the vibrancy and huge turnout of Yes support is not the main thing pushing unionism off the tracks. Unionist leaders are doing that quite nicely themselves.

Take Arlene Foster, leader of the Democratic Unionist Party.

While Boris Johnson has been brazening it out with Brussels, his Brexit deal has been unravelling “over by”. Foster handed Johnson a lifeline when she agreed to drop objections to regulatory checks in the Irish Sea, a measure she’d previously condemned as separating Northern Ireland “politically and economically” from the mainland.

All Under One Banner Independence March - Holyrood to Meadows. Picture: Pic Lisa Ferguson

Of course, her earlier analysis was quite right.

Under the “two borders” deal, folk from the Six Counties will rub shoulders with their southern cousins in the same “non-British citizens” queue each time they try to enter the UK. Indeed, for thousands of Ulster folk carrying Irish passports, the only thing distinguishing north from south in future will be their accents. Is that enough to maintain the enmity of ages?

Doubtful.

Yet this is the scenario “won” by the traditional advocates of No Surrender – the Democratic Unionists.

Foster believes she has won important powers for Stormont. But that also unites sceptical citizens across the “divide”. The Northern Ireland Assembly has failed to sit for three years and, despite its power-sharing composition, the two sides are locked in stalemate. With the demise of the Reverend Ian Paisley and Martin McGuinness, the era of Chuckle Brothers compromise has evaporated. So the idea that Stormont will adjudicate every four years on the future of their lives is a disaster in the eyes of the DUP’s most important supporters – farmers and business people.

Manufacturing NI has said the new “two borders” proposals are “worse than no deal” and will leave the agri-food industry “decimated”. Retail NI said they would introduce “costly and intrusive” checks, making Northern Irish goods “less competitive”, and the chair of CBI Northern Ireland said the proposals represented the worst of both worlds for business.

Add to that unprecedented chorus of agreement, the complete unanimity of Ms Foster’s political opponents. The unionists (UUP), loyalists (TUV), moderate nationalists (SDLP), republicans (Sinn Fein) and moderates (Alliance and Green Party) are all totally opposed to the double borders proposed by Foster and Johnson.

Meanwhile the long-awaited report into the non-domestic Renewable Heat Incentive (RHI), which crashed the Northern Ireland Assembly before folding amid allegations of corruption, will not now be released until November – conveniently tucked away after Brexit. Few voters will have missed this shameful sidestepping of responsibility by the former first minister who launched the RHI scheme in 2012.

Admittedly, the prospect of a border poll still looks distant and the mechanism to trigger one is fairly arcane. The Northern Ireland Secretary needs electoral (not just opinion poll) evidence of demand, so if no Protestant pro-reunification party emerges, voters could remain unhappy, disillusioned with the Union, horrified by Brexit – yet constitutionally stuck.

Could Foster’s unpopularity among her traditional supporters and the associated haemorrhage in support for the Union become a condition that loups the Irish Sea and infects the Scottish Conservatives too – especially now that Jackson Carlaw has taken the party in the same, unpopular “do or die” direction?

At the Tory party conference, the Eastwood MP unilaterally announced he would support the Prime Minister dragging Scotland out of the EU into a no-deal Brexit. Apparently there was “cold fury” among Holyrood Tories who had neither discussed nor approved the move.

The extent of this breach of faith was starkly revealed at First Minister’s Questions, when Nicola Sturgeon spoke about the Tories’ previous hostility to a no-deal Brexit. As their interim leader floundered, Ruth Davidson could be seen quietly nodding. The 21 moderates expelled by Johnson were the natural allies of Scottish Tories. But Carlaw has cast his comrades adrift with the same indifference to argument, party tradition and basic electability as his hard-line Conservative and unionist counterparts Foster and Johnson.

But unrest is not confined to Holyrood.

In Scotland, like Northern Ireland, traditional supporters are in quiet revolt. Farmers have already warned of “civil unrest” in rural areas in the event of a no-deal Brexit, as many are reliant on EU trade and could face expensive tariffs and loss of unique Scots brands in a no-deal scenario.

The UK Government apparently has a £500 million plan to support farmers by buying slaughtered lambs and other livestock if that occurs. But the plan only confirms the size of economic shock that Westminster is quite ready to administer – with the blessing of the farmer’s traditional party of choice, the Scottish Conservatives.

Ditto business. Last week a Fraser of Allander report revealed that nine in ten Scottish businesses expect the ongoing economic uncertainty of Brexit to have an important impact on their operations. Thirty per cent are scaling back all new investment and 71 per cent expect costs to increase.

Rattling core support has already impacted on the Scottish Conservatives fortunes. A recent YouGov poll recorded a 14 per cent slump in the Scottish constituencies they won after the 2017 snap election. Fifty-eight per cent of voters in these seats also backed a second EU referendum and 63 per cent would vote Remain if a second vote was held today, when don’t knows are excluded.

How on earth can the Scottish Tories expect to retain seats such as West Aberdeenshire and Kincardine if they alienate farmers, business and moderate Remain-voting Conservatives? Why don’t they seem to care? Does Carlaw appreciate that the health of the Union rests, in large part, upon the fortunes of his benighted party? Certainly, Labour and the Liberal Democrats intend to wrest that Union mantle back from the Tories before the next election.

The damage, however, has already been done.

In Scotland and in Northern Ireland, unionist leaders have shown they care more about alignment with an English nationalist Conservative leader – himself trying to stave off an even more extreme Brexit Party – than the economic or social interests of their own voters or most loyal supporters.

The siren call of southern paymasters has proved loudest for arch-advocates of the status quo on both sides of the Irish Sea.

This is how a union dies.

Lesley Riddoch: This is why Unionism is in terminal decline