The Scottish Labour spring conference was the party’s big chance to put forward the positive case for the union. No campaigners are clearly irritated by constant jibes from the other side that they have nothing positive to say about the benefits of the union. Conference was the best platform between now and September 18 to tell a story, not about the disadvantages of leaving, but about the advantages of staying. So what did they do?

 Well, first of all, there was a lot of playing the man – Alex Salmond – rather than the ball – Scotland’s constitutional future. Even when Ed Milliband did stop the Salmond bashing to try to make the case for the UK and the union, it was actually a case based on the benefits of Labour running the UK. The picture he painted was of a progressive union, built on what he described as Labour principles of justice, fairness and compassion. A vision of Britain under his premiership that would be so attractive, he hoped, that it would provide a compelling case for voting no in 2014 and then Labour in 2015. The problem with this as a case for the union, though, is that Labour might not actually win the next election.

 One of the stronger pro-independence cards from the Yes campaign is the ‘localist’ argument, that it would be always be better if decisions about Scotland were made in Scotland, regardless of who was in charge. This means both the radical left around RIC and the centre right around Wealthy Nation can see independence as an opportunity to advance their cause. Despite the white paper being very much an SNP vision, the wider benefits of independence set out by the Yes campaign do not require an SNP victory. This allows the Yes campaign to try to appeal to voters whose primary political affiliation is not to the SNP.

It is therefore hard for Labour to make a case for the union, based on what a future Labour Westminster Government could use its powers to do in Scotland, while simultaneously attacking the current Westminster coalition for what it is doing in Scotland with those very same powers. If the Labour case for the union rests on Labour always being in a position to run the union it is on pretty shaky ground.

This is a fundamental weakness in the Labour unionist case. Are they making a positive case for the union, even if the Tories are running the union, or a positive case for the party’s management of the union? Labour voters alone cannot win the referendum, and telling non-labour no voters that a no vote will mean the Labour vision of the union is unlikely to persuade them to actually come out and vote no.

The other problem with the Labour case for the union, as presented in Perth, is that it is impossible to make the case for further devolution in Scotland without discussing the impact on the rest of the UK. The Labour argument against a further devolution option on the referendum ballot paper was that, while independence or union was a choice people in Scotland could legitimately make for themselves, further devolution would be a decision for the whole of the UK. Scotland could not simply unilaterally demand more powers short of independence – the consent of people in England, Wales and Northern Ireland would be required. Ultimately, further devolution is an issue for Westminster, as the sovereign UK parliament, to decide.

This is the light in which Gordon Brown’s attempt at a positive case should be viewed. This is the clearest attempt by a Labour politician to make the wider case for a new set of UK institutions to address the concerns of people in Scotland. In his speech in Glasgow a couple of weeks ago he called for a range of measures, including a new constitutional law to set out the purpose of the UK as pooling and sharing resources, a constitutional guarantee of the permanence of the Scottish Parliament, the replacement of the Barnett formula with a new tax-sharing agreement and extra powers for Holyrood. However, these proposals instantly open up fundamental questions about the whole UK, not just Scotland. To achieve what is proposed would require the end of the principle of UK parliamentary sovereignty – the notion that a current parliament cannot bind a future parliament – and its replacement with a very new kind of Constitutional settlement. That would be the only way to ensure the Westminster Act establishing the Scottish Parliament couldn’t simply be repealed. If we replaced the Barnett formula with a tax sharing agreement, who would the agreement then be between? Who would speak for England? If Westminster was expected to play this role, the case for a separate English Parliament would grow very compelling – not least because as Tam Dalyell pointed out in his ‘West Lothian question’, this kind of arrangement would mean Scottish MPs would have a say in England’s bargaining position on these tax arrangements, but not in Scotland’s. In terms of Brown’s vision of a solidarity union, would there actually be a consensus across the UK that this should be the core of what a renewed union would be about? Would it be Gordon’s vision of the union that triumphed – or David Cameron’s – or even Nigel Farage’s?

If Gordon Brown had really wanted to set out a new constitutional vision for the UK he could probably just have dusted down his old copy of the Charter 88 manifesto, which called for a bill of rights, PR, abolition of the Lords, and a written constitution that would include measures to “guarantee an equitable distribution of power between the nations of the United Kingdom”. But this would rather beg the question why he didn’t deliver all this as the UK Prime Minister with a parliamentary majority.

Labour’s biggest challenge here is to find a way to make a positive case for the union that will appeal to the voters it has lost to the SNP in Scotland, while ensuring floating voters in the south of England switch from the Tories to Labour rather than to UKIP. Describing the purpose of a renewed union as pooling and sharing resources may well sound to voters in marginal consequences in Kent like a requirement that they permanently subsidise the poorer regions of the UK. Will the federal settlement required by Gordon Brown’s approach, implying some kind of English Parliament, appeal to voters in England? Labour are still scarred by the rejection of the North East Assembly back in 2004.

Hence the fact that Labour were left with three key messages in Perth – you can’t trust Alex Salmond, this is the best union in history (so why wreck it) and hang on for a Labour victory and it will all be alright. But while Johann Lamont clearly despises Salmond, it is not clear that voters do to the same extent. Turning the referendum into a Lamont vs Salmond attractiveness contest may not be the wisest choice. Also, given widespread support for more Holyrood powers on specific issues among current no voters, it is not clear that they see the existing union in the same glowing terms. Furthermore, if the UK polls continue to narrow – the latest poll in the Times gives Labour only a 1% lead – it is not clear that voters will have enough faith in the certainty of Labour victory for ‘Ed will save us’ to work either.

 Ultimately, Labour are going to need to find a positive case for union that does not rely on Labour being in charge – and opportunities to make this case are running out.