Ed Miliband's argument that independence for Scotland would cause a "race to the bottom" is a curiously narrow one.

It highlights the SNP's corporation tax-cutting policy as the most significant likely outcome of a Yes vote in September's referendum.

Mr Miliband and Labour are keen to question Alex Salmond's progressive credentials, and are entitled to do so.

Loading article content

But the argument risks seeming like more of what pro-Unionists have been criticised for - at best a negative and, at worst, a scaremongering message.

Many of those considering voting Yes are not doing so because they fear a race to the bottom, but because they aspire for a better Scotland, a more socially just and equal society which they do not see coming from the Union and from a politics still dominated by Westminster.

Those who believe Scotland could be an independent country with a strong social security system and public services, and a thriving economy on the Nordic model, are not contemplating a race to the bottom. Instead, they see this country leading the way to a better politics in the UK.

Mr Miliband's speech in Perth yesterday began to engage more seriously with those aspirations. He says Labour can deliver social justice across the UK. This assertion matters because he is a very significant figure in the referendum's outcome.

It is worth recognising voters who are core Labour supporters will be key to the result in September.

Success for the Yes campaign will depend on persuading enough traditional Labour voters their concerns are likely to continue to be ignored by UK parties seeking to win General Elections by satisfying Middle England.

So what Mr Miliband does, says and thinks is crucial. A credible alternative in terms of proposals for increased devolution would help. It is not yet clear whether the alternative on offer from Labour meets this definition.

However, even more important is Mr Miliband's own credibility. Despite making some astute political calls about issues such as phone hacking, the BBC, globalisation and the economy, the Labour leader struggles to persuade voters he is a realistic Prime Minister in waiting. His personal ratings in Scotland remain higher than in England. But that is not the issue.

Labour supporters are being urged by many sides, including former MP Jim Sillars and those behind the Common Weal project at the Jimmy Reid Foundation, to consider a Yes vote in the hopes of a revitalised left in a future independent Scotland.

It is not clear how many stand to be persuaded of this, but one thing is clear. If Mr Miliband looks as if he can realistically win the 2015 General Election for Labour, then many of those core Scottish supporters will be strongly inclined to hang on for that outcome.

But if he continues to struggle to convince, the suggestion Labour voters should consider a Yes vote may well have more traction.

For this reason Mr Miliband's approach in the coming months is likely to take on very considerable significance.