Imagine it’s 19 September 2014 and Scotland has voted No. If that does prove to be the case the No side’s victory will have been built on a two-pronged strategy.

The first prong is the approach taken by the UK Government in rejecting a Scottish Government vision of independence that would involve continuing partnership with the UK on currency, financial and energy markets, broadcasting and many other matters. The intention is to deter voters from saying Yes by making independence appear as risky and difficult as possible.

The other part of the strategy, led by the pro-union parties, is the carrot to go with the UK Government’s stick, and that is to offer more devolution to Scotland if Scots vote No. The message is ‘vote No and get more’. It is aimed at the large middle ground of the Scottish electorate that has no great faith in the UK Government to look after Scotland’s interests, but is reluctant to go the whole hog of national independence.

So what is the offer of more devolution? Well first of all there isn’t a single offer. The Liberal Democrats and Labour have each already published their proposals on more devolution, the Conservatives will follow suit next month, and a number of pro-union think tanks have added their two-penn’orth. These proposals reflect different traditions and priorities on devolution but nonetheless contain a number of common themes. Let me name four:

1. First, to give the Scottish Parliament stronger protection against the theoretical possibility that Westminster could abolish it.
2. Second, to give local government in Scotland additional powers, some passed down from Holyrood, some direct from Westminster. Labour has the fullest set of proposals on local government powers covering economic development, training and skills, taking control of the UK Government’s (welfare to) Work Programme and the prospect of special powers for Orkney, Shetland and the Western Isles
3. The third theme is to give the Scottish Parliament powers over some aspects of welfare policy, covering in the various proposals attendance allowance in the social care field, housing benefit and the Work Programme (which Labour would pass on further to local government). None envisage devolution of the most expensive items in the welfare field like pensions and incapacity benefits.
4. The final field has had most attention and that is to give the Scottish Parliament additional tax powers.

The headline issue here has become the proportion of the Scottish Parliament’s spending that would be covered by its own tax revenues.

The most radical proposal was put forward back in 2012 by the Reform Scotland think tank, which recommended the Parliament should have tax powers to cover two-thirds of its spending. The Liberal Democrats have since urged everyone to commit to a common position that future tax powers should raise enough to cover at least half of the Parliament’s spending.

That is a hurdle that Labour’s proposals, published in March 2014, fail to meet. Labour’s Devolution Commission has set a ceiling of 40% of the Scottish Parliament’s spending to be covered by its own tax revenues, arguing that 60% needs to come as a grant from Westminster to underline Scotland’s place in the union and a continuing commitment to shared services and solidarity across the UK.

None of the other parties or think tanks have taken this approach of defining obligations to the union first and framing what tax powers Scotland might have as a secondary step and, in a sense, a subsidiary priority. That approach has come under attack, not least by Reform Scotland, whose number-crunchers maintain that only a quarter or so of the Parliament’s spending – and not the widely trumpeted 40% - would be covered by Labour’s proposals.

Whatever the precise figures, it seems that Labour’s proposals are the outcome of an internal party compromise between the Holyrood and the Westminster parts of the Labour Party, with latter more cautious and winning out.

We await the publication of the Conservatives’ proposals next month, but it’s quite possible that they - a party widely seen as sceptical of devolution - may produce proposals on tax devolution that are more far-reaching than Labour’s. 

What to make of all this? Well, there is no consensus on what more might mean, in particular on tax devolution. We can anticipate that in the event of a No vote, the three pro-union parties will each have some kind of manifesto commitment on more devolution at the May 2015 UK election. Whether they can agree on the extent of more devolution both between and within parties is less clear.

There is a risk in this that expectations will be raised that will not easily be met – even now there is some scepticism in public opinion that the promise of more devolution will be delivered. There will likely be only one winner if expectations about more devolution do get raised that aren’t met: the SNP.

Wouldn’t that be an irony: Scotland votes No and the SNP ends up the winner!