AFTER the Brexit ding-dong, comes the Scotland ding-dong.

Last week, Ian Blackford and Boris Johnson engaged once again in the less-than-merry constitutional dance over indyref2 with the former demanding a second vote on Scotland’s future and the latter saying go whistle.

For good measure, Michael Russell, the Scottish Government’s constitutional supremo, travelled down to London to engage in another Groundhog Day experience, using the same arguments, which Alister Jack, the Scottish Secretary, duly rebuffed with the same responses.

In his interview with The Herald today, the SNP leader at Westminster argued that the “dam will burst” and the political pressure – possibly helped by Scottish Labour at some point - will force the Prime Minister to cave in sooner rather than later.

But it seems impossible to imagine Mr Johnson, having spent, like the rest of us, three years of turmoil over Brexit, having won the election with an 80-seat majority to “get Brexit done” and looking ahead to try to eke out a trade deal with the EU at breakneck speed, would turn round and concede to having another poll that will be deeply divisive.

The PM is the self-styled Minister for the Union. He does not want to become, just nine months after taking office, the Minister who lost the Union.

So, a poll on Scottish independence in 2020 seems completely out of the question.

Mr Blackford argued that it was undemocratic for the PM to face down the election result in Scotland when his party won 47 out of the 59 Scottish seats and cited the Claim of Right as an argument that sovereignty rested with the Scottish people and not with Westminster.

But the realpolitik is this does not wash with Whitehall.

The General Election was a UK poll. Mr Johnson stood on a manifesto not to facilitate indyref2 and won a landslide.

The PM’s point is that however much the Nationalists want Scotland to be independent - and act as if it already is, barring the technicality of a vote to realise it - the nation remains part of the United Kingdom.

The SNP understandably frame all their political arguments within the context of Scotland; equally, the Conservatives frame theirs in the context of the UK; in each both act on firmer ground.

Mr Blackford revealed he had been told privately by unnamed Cabinet ministers that they too believed the PM’s “obstinate” opposition to indyref2 would ultimately crumble.

Two-thirds the way through The Herald interview, the SNP leader conceded that if Mr Johnson did continue to dig his heels in, then indyref2 would be the backdrop to the 2021 Holyrood election. It would, in effect, become a referendum on a referendum.

All it seems the pro-independence forces can do is to build their support between now and 2021 to such a level that the PM’s argument collapses in on itself. The pro-independence protest in Glasgow is set to be replicated throughout Scotland on seven more occasions. There is even talk of bringing a pro-indyref2 march and rally to the streets of London.

Should Nicola Sturgeon repeat Alex Salmond’s remarkable victory of 2011 when the SNP won an overall majority in the Scottish Parliament, then the pressure on the PM would build considerably.

And yet even then the Conservative leader could insist seven years is neither a generation nor a lifetime and continue to cock a snook at the Nationalists.

One unknown ingredient into the political mix will be the fall-out to the Salmond trial in March and what, if anything, it could mean for SNP fortunes.

But regardless of that, the spectacle of the constitutional tug-of-war between the Nationalists and the Tories looks set, in the near future at least, to go on…and on…and on.