WHATEVER you make of the SNP’s House of Commons protest, the blood definitely seemed to be pumping round the veins of the Yes movement at the weekend. Ian Blackford, the nationalist leader at Westminster, said – with the kind of certainty that sometimes comes over politicians at moments like these – that history would remember the walkout as a defining moment. Pro-SNP commentators said something similar: nothing will ever be the same again, this is a turning point, a pivotal event, a game changer for the SNP and the campaign for independence.

But if the nationalists’ walkout – staged during Prime Minister’s Questions over the EU Withdrawal Bill – really is a game-changer, in what way is it different from the referendum White Paper? Or the Growth Commission report? Or the polls four years ago showing the Yes campaign ahead? Or the “surge” in membership after the referendum? Or the “surge” in membership last week? Or the moment when this or that person changed his or her mind and decided they now supported independence? All of these moments have also been declared as game-changers for the SNP and yet here we are still playing the same game, with many of the same players, and most of the same rules.

Writing in The Sunday Herald yesterday, Mr Blackford’s take on the situation was that, in walking out of the Commons, he and his colleagues were standing up for Scotland in Westminster. The country was in the middle of a constitutional crisis, he said, devolution was being dismantled, and the SNP had a duty to stand up to a power grab. He also highlighted the fact that, as a result of the walkout, the Speaker had granted today’s debate on the Withdrawal Bill and devolution. All in all, Mr Blackford could hardly contain his satisfaction at a job well done, and his delight that Westminster had taken a big old Scottish punch in the face.

However, as usual with the SNP, it’s important to be extremely wary and to remember the difference between asserted facts, angrily and passionately delivered, and the situation as it really is. In recent years, voters have become increasingly sophisticated at spotting this difference, and at reading the nationalists’ motives and actions, which is probably why their support has been falling. But the Commons furore over the last few days has mixed things up thoroughly, with a view to achieving the maximum effect – with a view, in fact, to finding that elusive game-changer.

On the SNP’s substantive point, everyone, apart from the Tories, agrees: the EU Withdrawal Bill undermines one of the founding principles of devolution that Holyrood has control over devolved areas, and the UK Government should concede the fact. It will make negotiating post-Brexit trade deals difficult and could give the SNP the power to cause endless trouble, but that’s something the UK Government is just going to have to put up with.

The SNP also deserves credit for making its point about the Withdrawal Bill in an effective and dramatic way. There’s a long tradition of direct protest in the Commons, from Michael Heseltine, John McDonnell and others interfering with the mace, to flour being thrown at Tony Blair. In fact, as protests go, the SNP’s was rather tame – in the 1930s, the Labour MP John McGovern’s refusal to sit down led to a brawl between Labour MPs and the serjeant-at-arms. So the SNP was really only doing what has always happened in the Commons. The only thing we could have done without was the self-satisfied selfie afterwards.

However, that, I think, is pretty much all we can say about the SNP walkout before the facts and the assertions start to diverge. For instance, the SNP MP Tommy Sheppard was still asserting yesterday that the walkout was instinctive and improvised even though it seems clear that Alex Salmond suggested the protest to Ian Blackford the night before (Mr Salmond likes to claim credit for great political events – he did the same with the fall of Margaret Thatcher and who knows what else he has achieved behind the scenes).

Many weary voters will also be able to spot the real motives at work in the SNP walkout. Mr Blackford says it’s all about fighting Brexit and its consequences, but we know that it could be in the SNP’s interests for Brexit to go ahead and fail – in fact, in private, many SNP supporters hope and pray for a disastrous Brexit because they think it will help to achieve what they really care about, which is independence. To that extent, the SNP walkout wasn’t about thwarting the Withdrawal Bill, it was about achieving independence – it was a selfish act dolled up as a blow against Brexit.

The only problem for the SNP is that the protest is unlikely to work and take them closer to independence. For a start, many voters are not going to believe the hysterical language that devolution is being demolished – the history of devolution just doesn’t bear that out. The other problem is that the SNP’s anti-Brexit stance no longer fits easily with other elements of party policy, particularly the pro-austerity Growth Commission, making the whole message look incoherent. Faster than you could imagine, the dreaded Growth Commission report has become like anti-matter for the SNP, refusing to co-exist with the party’s self-proclaimed socially liberal aesthetic.

Labour has obviously spotted this flank opening up and has sought to exploit it as much as it can. In a speech to the Fabian Society at the weekend, Gordon Brown said the SNP could no longer claim to be the party of anti-austerity, leaving Scottish Labour as the true party of social justice.

That’s a claim the nationalists will dispute of course, but as Mr Brown stomped from one end of the stage to the other, many in the audience will surely have realised he has a point. With its right hand, the SNP has delivered a punch to Westminster, but with its left hand it has delivered a self-inflicted wound by undermining a progressive agenda that still has a chance one day of winning a referendum – more chance, certainly, than a bit of amateur dramatics in the House of Commons.