I ask these questions because rather like we understand appealing to patriotism is the last refuge of the scoundrel in political debate, so it always seems to be that, when our politicians are faced with a crucial decision, the threat of Brexit to the Union is rolled out in expectation of some Pavlovian response that will ensure no dissent is possible. Fortunately there are enough MPs who have minds of their own, who challenge baseless assertions and can conclude if they have any merit.

The Scottish Secretary, David Mundell, has yet again warned that leaving the EU on Friday without a deal will strengthen the hand of nationalists. Well, he would, wouldn’t he? Along with David Cameron, Ruth Davidson and John Major, Mundell claimed during the 2016 referendum that the UK leaving the EU could be a threat to Scotland remaining in the UK, on the grounds that, were Scotland to vote differently from the rest of the UK, the clamour for Scottish independence could become much strengthened, even irresistible.

There is no doubt the SNP has tried to make Mundell’s prophecy come true by turning the difference between attitudes of voters in Scotland and those in England (and Wales) to its advantage. Nicola Sturgeon has incessantly stamped her feet with indignation, jetted around European capitals and never stopped grievance-mongering on TV sofas or at marches and rallies that provided a platform. Yet for all the First Minister’s mind-numbing histrionics (or maybe because of this approach), the polling over the last two years has moved gradually – and indisputably – against the nationalists.

The Scottish Secretarys proposal is playing into the SNPs hands but hes too blind to see it, warns Brian Monteith. Picture: Jeff J Mitchell/Getty Images

This was illustrated only the other week when a poll commissioned by a nationalist think-tank found that support for independence had slipped below the totemic 40 per cent for the first time in five years to 38 per cent – while support for remaining in the UK had risen to more than 62 per cent. These figures of confidence in the UK are all the more encouraging because they have come after the British Government has made such a disastrous job of negotiating an orderly departure from the EU – for which Mundell takes his own share of the blame.

The reality is that, rather than Brexit threaten the Union, as Mundell was encouraging us to believe, voters are able to make rational decisions and vote for different parties or propositions in different elections and referendums after weighing up what is in their interest. There are Europhile and Eurosceptic nationalists just as there are Europhile and Eurosceptic unionists and such voters have repudiated those politicians such as Sturgeon and Mundell who play on their nationalist or unionist sympathies and have taken their votes for granted.

Sturgeon does not have the right to now claim the support of Scots unionists who voted to remain in the EU for Scottish independence. Indeed some voters get very annoyed when she uses their vote to “remain” in the EU to justify her cause – not least because, had her Yes campaign won in 2014, Scotland would already be out of the EU and more than 140,000 EU citizens would have had their rights to stay in Scotland revoked.

Even though Brexit has clearly not shifted opinion towards independence in Scotland, there is no room for complacency among unionists. There are still threats to the Union that need to be evaluated and responses considered.

In regard to Brexit, the prospect of MPs backing the deal between the EU and Prime Minister contained in the Withdrawal Agreement and its accompanying Political Declaration presents a threat to the cohesion and survival of the Union.

By treating Northern Ireland differently from Scotland when residents in both voted for the UK to remain in the EU creates a grievance that the SNP will seek to exploit. When a border between Northern Ireland and the rest of the UK appears down the Irish Sea, that grievance will become tangible and immediately weaponised, with demands that Scotland should have been treated differently too. That is why MPs who believe in the unity of the UK must vote against the Withdrawal Agreement unless it is significantly changed by removing or time-limiting its backstop protocol.

The DUP MPs understand this. That some Scottish Conservative and Unionist MPs cannot see the danger has the potential to cost them dear politically and rupture the Union. The sacrifice that the Prime Minister’s “deal” makes of fishing ports by including fishing negotiations in the transition period will simply add to the political pain in seats the Tories won in 2017.

Fortunately the likelihood of the Withdrawal Agreement passing in the Commons remains low – unless something helpful comes out of the negotiations with Labour. The Commons will then face a challenge: vote for an extension of EU membership, to 30 June or into 2020, or leave with no deal. To help convince MPs they should back May in seeking an extension so that she might eventually get her deal approved (for that is her sole purpose in asking for more time), Mundell is putting the Union at further risk.

There can be no doubt that if there is a further extension – of whatever length – the possibility of there being a second EU referendum increases. The SNP calculates that in another EU referendum Scottish voters will again vote differently from England and Wales in backing membership, but in greater numbers than in 2016. This larger political gulf will then be used to claim the time has, after all, come to leave the UK. This time it might work.

For Unionists, leaving without the Withdrawal Agreement is the better course – the whole UK leaves together and every constituent part is treated the same. The preparations on aviation, medicine and borders etc, and mini-deals to make them work, are in place. The Article 50 extension Mundell proposes plays into the SNP’s hands but he is too blind to see it.

l Brian Monteith is a director of Global Britain

Brian Monteith: A further Article 50 extension is a threat to the Union