The Scotland Bill – debated for the last time in the House of Commons today – was always intended to recalibrate Scottish politics.

Nationalists viewed it as a “trap”, and while Unionists obviously aimed to make life difficult for the SNP there was also a genuine desire to move discourse away from grievance and onto less constitutional territory.

Last week I sensed this was finally working. Uncharacteristically, the Scottish Government appeared wrong-footed by Kezia Dugdale’s commitment to mitigating forthcoming tax credit cuts, finally stumbling towards a coherent position on Thursday.

Claiming that it doesn’t have the “power” to do so is no longer an option, for today Labour will follow Gordon Brown and the Daily Record in declaring that “the Vow” has “been delivered in full”. Even the SNP appears to agree, saying – in a marked change of tone – that the UK Government’s “belated concessions” were “welcome”.

It’s been a hard slog for Scottish Secretary David Mundell, often in the face of institutional opposition from the Treasury and Department of Work and Pensions, and it isn’t over yet: there’s still the House of Lords to clear and a “fiscal framework” to agree, but all going well the Scotland Bill should become an Act next March. Ministers are also hopeful Lord Smith will soon give it his seal of approval.

The timing, of course, is deliberate, for Royal Assent will be granted just weeks before Scotland goes to the polls with a raft – current or forthcoming – of new powers. “The realisation of additional welfare responsibilities will start to bite,” says a senior UK Government source. “The issues will start stacking up, and a lot of people the SNP have kept on board by criticising the UK system will become a lot more difficult to satisfy.”

The Conservatives also believe the events of last week – in which Social Justice Secretary Alex Neil performed a messy U-turn on whether or not the Scotland Bill granted the power to mitigate tax credit cuts – highlighted “capacity” issues. “They’re trying to take over welfare responsibility with half a dozen officials,” adds the source, “and it’s a complicated area.”

Thus the Scotland Bill and Scottish Labour’s recent conference has upset the twin dynamic at the heart of Scottish politics, or rather the SNP’s continuing dominance of Scottish politics: a delicate balancing act between maintaining grievance (usually on points of process) and governing well, and the related habit of talking left while generally legislating in a pragmatic way.

Until last week the SNP had the upper hand in both respects, but after Wednesday the political struggle is a more equal one. There had clearly been an internal debate about how to respond, between fiscal conservatives like Finance Secretary John Swinney (who’d gone out of his way to play down expectations) and Nicola Sturgeon, the latter acutely aware she’d have to end up matching Kezia Dugdale’s policy “offer”.

I suspect the SNP wanted to resist the shift in dynamic for a little longer, for of course to retain a majority after next year’s Holyrood election it needs to keep “the 45” on side while the three Unionist parties (and to an extent the Greens) slug it out over “the 55”. An election fought on the basis of parties and personalities suits the Nationalists; a campaign based on “record” and policies does not.

And although Scottish Labour has been clever in its framing of the tax credit pledge – explicitly linking it to the SNP’s commitment to cut Air Passenger Duty – by moving so early it gives the Scottish Government time to do what it does best: triangulate. And if that happens following the Comprehensive Spending Review later this month, then Ms Dugdale could find herself struggling to take any credit.

That said, triangulation comes at a cost, and given how tight the Scottish budget already is, I don’t envy Mr Swinney having to find a few hundred million pounds to “top-up” tax credit cuts. Next year he’ll also have to set the new “Scottish rate” of income tax, so an indelicate move on any of those fronts could undermine the SNP’s broad coalition of electoral support. And given this election will be fought on the “list” vote like never before, a credible challenge from either the right (Scottish Tories) or left (Greens, RISE, etc) is something the SNP realises it has to guard against.

Key to which is Middle Scotland. Contrary to the rhetoric, the SNP hasn’t won every election since 2007 by being left wing but by promising “free” stuff for the middle classes, but with more and more of its budget taken up mitigating Westminster cuts, obviously that becomes much harder to maintain, especially when coupled with a deep reluctance to increase income tax. Not only will scrutiny be greater than ever before but the Scottish Government’s room for manoeuvre much narrower.

Of course another presidential Holyrood election – as in 2011 – will help the SNP: Scottish Labour realise that if May’s ballot is framed in terms of Nicola versus Kez then the former will win hands down. But given that Labour will fight this election with no realistic expectation of actually winning, that frees it up to make life as difficult as possible for its Nationalist opponents.

Only, however, until 6 May 2016, after which the independence question looks likely to reassert itself (especially if Labour suffers further losses). What I’ve called the “impatient” wing of the SNP – those who believe a second referendum should happen sooner rather than later – are already on manoeuvres, with Alex Salmond and others today attempting to amend the Scotland Bill to give Holyrood full control of indyref2.

But while the First Minister might recognise the logic of their position (that circumstances might never be this good again) she doesn’t want it clouding next May’s election. Instead the “cautious” or “R2D2” contingent remains the majority, viewing “referendum 2, defeat 2” as too much of a risk to the independence movement and the SNP’s continuing electoral prowess.

After polling day, however, Ms Sturgeon and others will be content to wait and see, while also taking time to develop a more coherent currency option, considered crucial to winning indyref2. But they can’t wait too long. The political cycle is bound to work against the SNP eventually, and if it gets to May 2021 without “material change” sufficient for a fresh independence push then the decisive moment might pass.

If the European referendum turns out to be a red herring (with Scotland and the rest of the UK voting decisively to stay “In”) then it’s easy to see the 2020 general election becoming the touch point, with the perfect storm of a recovered oil price, George Osborne beating Jeremy Corbyn to Downing Street and the usual arguments against independence becoming subject to the law of diminishing returns.

Despite last week’s developments the SNP remains in its imperial phase, impervious to attack and not seriously ruffled by a more confident Scottish Labour Party. Nevertheless the ground has shifted – however marginally – and all great empires (Nationalist or Unionist) must eventually fall.