AMID the blue on blue bloodbath, the subject of Scotland and the future of the Union has not featured highly in the In-Out debate but that is about to change.

Next Thursday, Nicola Sturgeon will appear in a live two-hour debate on ITV and it is believed she will be up against no less a Brexiter than Alexander Boris de Pfeffel Johnson.

The high-profile appearance by the First Minister in a set-piece head-to-head will thrust into the spotlight the notion that the vote on June 23 is not just about the membership of one union but two.

While David Cameron, the head of the Remain campaign, has thrown himself headlong into defending the UK’s place in the EU, he has only tentatively acknowledged a Brexit fall-out could once again push the issue of Scotland’s place in the UK to the fore.

In the game of euro poker, the Union card has still to be turned over and played; the reason it has not been already is that the issue is double-edged.

Mr Cameron genuinely believes the Scottish question was answered on September 18 2014. That was it for a generation; done and dusted until at least 2040.

Yet if opinion polls continue to show the EU contest is too close to call and the PM were forced to play the Union card, he would be acknowledging that the issue of Scotland and the Union has not been settled in the way he had previously claimed.

Thus far, while the Remainers have been reluctant to touch on the Brexit-Scexit scenario, the Leavers have simply brushed it aside.

Michael Gove, the leading Leave campaigner, has boldly claimed most of his fellow Scots will back leaving the EU despite the fact that all polls to date have shown Scots are far more likely to support Britain staying in the EU, particularly older voters.

Last month, the UK Justice Secretary said: “When we vote to leave, it will be clear that,having voted to leave one union, the last thing the people of Scotland will want to do is to break up another.”

Last week, his cabinet colleague and fellow Outer Chris Grayling was also dismissive. He suggested it was simply not sensible or feasible for Scots to start talking about a second independence poll, noting how Alex Salmond had made clear to the country that the 2014 vote would last for a generation and that people should hold him to that.

The Commons Leader also stressed the vote later this month was a UK vote and it should be on that basis that the result is accepted; in other words, if there is a vote to leave, Scots should just accept it even if Scotland by a majority votes to stay in.

Meantime, the high priest of Europhobia Nigel Farage has dismissed the notion of a second independence poll in his usual idiosyncratic way.

“With oil at 45, 50 bucks a barrel, it's moonshine,” declared the Ukip leader, adding that, by the way, an independent Scotland within the EU was an oxymoron.

However, Mr Salmond repeatedly insists that if there were an attempt to drag Scotland out of the EU against its wishes, then this would represent the “material change” necessary for Ms Sturgeon to call a second independence poll and the Prime Minister could not gainsay a Holyrood vote to that effect.

Despite his previous “once in a generation” declaration, the Gordon MP argues circumstances have changed and his successor has been given a mandate to seek another referendum because of the electoral mandates last year in the UK election and this year in the Scottish one.

His colleague Angus Robertson, the leading SNP torch-bearer at Westminster, made clear during one recent PMQs that a Brexit vote against the wishes of the Scottish people would lead to a “demand” for another vote on Scotland’s future.

And yet the FM is more wary.

She blows hot saying that a Brexit vote would lead to a mood swing in Scotland and that there would “almost certainly” be, in such circumstances, a second poll.

She blows cold in making clear there would have to be a sustained public demand for another vote, acknowledging she has to make “better arguments” to persuade a majority of Scots towards independence.

Interestingly, last month a poll showed most people in Scotland did not want a second independence poll should Scotland be taken out of the EU against its will; the numbers were 48 to 44 against.

And in a significant move last week, Humza Yousaf, the Scottish Government’s transport minister who sits within Ms Sturgeon’s inner circle, took a decidely different tack to Mr Salmond, making clear that he was opposed to any rush to a second poll on the back of Brexit.

Admitting that a vote to leave, if Scots had voted to stay, would precipitate demand for another independence referendum, he made clear he, personally, would not want one in such circumstances.

“It makes the argument for independence very difficult…,” he said, adding: “It presents us with some additional difficulties and some additional challenges." Frustratingly, he did not elaborate what they were.

As the opinion poll last month showed the assumption by some gung-ho nationalists, that a Brexit vote would automatically lead to a groundswell of Scottish opinion demanding a second referendum, is not a given.

And what Ms Sturgeon and her colleagues have to be before calling another vote is as near as certain as they can be that they are going to win it. Another loss, Quebec-style, would truly put the Scottish question off the agenda for a generation.

The quandary for the SNP leader is that while the political momentum remains with her party for now, politics is so volatile that this might not always be the case; the window of opportunity may not be open for too long.

The quandary for the PM is more immediate; with less than three weeks to go, if the EU opinion polls show the In-Out contest is too close to call, he might well have no choice but to play the Union card. The $40,000 question is whether or not it will turn out to be the winning ace.

Equally, as voters take against the continuous flow of scaremongering and negativity, it might be the arrival of Ms Sturgeon on the scene with a more upbeat, positive pro-EU message, which could help swing the vote for Remain and give Mr Cameron a winning hand; helping him survive in Downing Street. The irony would be lost on no one.