Emran Mian is Director of the Social Market Foundation

On the front page of the Scottish Daily Record yesterday, Prime Minister David Cameron made a vow – along with Ed Miliband and Nick Clegg – to implement “faster, safer and better” change for Scotland than what independence would bring.

In one sense, this is no surprise. The referendum is two days away and too close to call. Signing a pledge is cheaper than risking a break-up of the Union. What’s more, Liberal Democrats are avowedly federalist; and it was Labour that started the process of devolution for Scotland in the first place. Yet it is the Prime Minister – and his party – that now lead the charge.

In May, the Scottish Conservatives published the report of the Commission on the Future Governance of Scotland. While Labour was promising only to extend by a little way more the Scottish Parliament’s tax raising powers, the Conservatives promised the maximum extent of devolution: the authority to set not only the rates of income tax but also the bands. The report says that the purpose of this is to close the ‘fiscal gap’, the difference between what the Scottish Parliament spends and what it raises in taxes, not to zero but to a point where Scottish politicians can no longer blame Westminster for public spending constraints.

While the principle invoked is of fiscal responsibility, high ground that Conservatives like to claim, the politics seem a bit naïve. If the Scottish Parliament hikes taxes then it has the money it gets from Westminster via the Barnett formula – the operation of which was also guaranteed by the Prime Minister in today’s vow – and what it raises on top. However, it is far from clear that the SNP – currently the majority party in the Scottish Parliament – will raise taxes above those of the UK.

Alex Salmond for one recognises perfectly well that this may reduce Scotland’s competitiveness, not least vis-à-vis the rest of the UK. His signature tax policy may well be the opposite one - already he has talked about reducing corporation tax in an independent Scotland. By the same logic he could take an aggressive stance on income tax, if that is what he controls in a new devolution settlement, and then the question for Westminster is a much harder one: whether to reduce what Scotland gets by the Barnett formula or to leave the formula as it is and effectively subsidise a low tax nation.

Westminster Conservatives in particular will ask a second question at that point: why not match Scotland’s move to higher competitiveness? Scotland’s case may even suggest that lower tax rates don’t reduce revenues, or by less than expected, because they lead to less avoidance and more economic activity.

But while the pledge on tax powers for Scotland may have this silver lining, at least for some Conservatives, the vow on further devolution stores up two other major challenges.

The first is that the negotiations over more devolution will take place while the political parties are setting themselves for the 2015 general election. The timetable proposed by Gordon Brown is to have a draft Scotland Bill ready by Burn’s Day at the end of January. Promising greater devolution within a federalist settlement is relatively straightforward for the Liberal Democrats. Labour has a strong base in Scotland and party strategists will recognise that they need to stay at the forefront of delivering more devolution if they’re to hold on to their seats as part of a wider push for a majority at Westminster. Conservatives however, will be under unique pressure to explain what further powers for Scotland mean for the rest of the UK, England in particular. This may mean following through on what George Osborne has begun to talk about for cities in terms of greater decision-taking, spending and potentially taxing powers. The units of a new federalist state in the Conservative version that emerges over this winter in the wake of a No vote may be Scotland, Manchester, Birmingham and London rather than Scotland, England, Wales and Northern Ireland.

The second challenge ahead is that the Yes campaign will regroup. For now there has been no serious critique made by separatists of the details of ‘the vow’ made by the unionist parties - they are focused on making the case for independence rather than giving credence to the alternative. That will change in the event of a No vote. During the referendum campaign the SNP has won over voters to the Yes side from the other parties, as well as people who don’t typically vote or are voting for the first time, and its ambition for the 2015 general election will be to convert the new Yes voters into SNP voters. Therefore it will take the terms of the vow as a starting point and campaign for one or more iconic measures that go much further. More tax powers, perhaps, extending the autonomy of the Scottish Parliament beyond just income tax; a sovereign wealth fund created from oil revenues to be placed under the control of the Scottish Government; or a nuclear-free Scotland with Trident to move away from Faslane. It may also be that they press more comprehensively for a new Scottish constitution that gives expression to the renewal of the nation; and at the very least for 16 and 17 year olds, enfranchised for the referendum, to retain the vote for the general election and  ever more.

All three main Westminster parties will have to decide how to respond to such demands while figuring out how best to engage with disgruntled English voters who may be intensely sceptical about giving Scotland special privileges.