THE setting was a spin-doctor's dream:

a smart, modern office suite with a floor-to-ceiling view over the Sir Chris Hoy velodrome in the heart of the new Emirates Arena Commonwealth Games venue. Pens and pads were laid out on tables arranged in a large square and posh sandwiches and fresh fruit awaited as members of the Shadow Cabinet arrived for their lunchtime meeting in Glasgow yesterday. At the other side of the venue Labour leader Ed Miliband was conducting a series of broadcasting interviews and, perched in the seats overlooking a pristine, stripey-blue indoor running track, one with the The Herald.

The impression was slick and professional: everything a political party wants to appear. For once, though, this wasn't the main image Labour wanted to serve up to the media. That was to be found 15 miles or so down the M8 in Motherwell, in a squat, concrete box of a building to be precise, nestled among the tower blocks of an ordinary 1960s housing scheme which until the last Scottish election would routinely have been labelled a "Labour heartland".

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It was there in the Isa Money Centre (named after a local Labour councillor and home to "OAP dominoes and martial arts classes," according to North Lanarkshire Council's website) Mr Miliband, Scots Labour leader Johann Lamont and Shadow Scottish Secretary Margaret Curran met the public. And not just any section of the public. They faced questions from people who the polls are increasingly suggesting could play a decisive role in September's referendum: traditional Labour supporters who are undecided on the issue of independence.

Mr Miliband's pitch to them is straightforward: Labour, not the SNP, is the true party of social justice, he said - just look at our plans to raise the top rate of income tax to 50p, to freeze your energy bills and, officially unveiled yesterday, to crack down on exploitative zero-hours contracts. The best, indeed the only chance of delivering such a programme, he insisted, is by remaining in the UK and electing a Labour Government in next year's Westminster election. He was very confident, he stressed, that Labour was on course to defeat a Tory party "in retreat".

The importance of Mr Miliband's mission cannot be overstated, and Labour knows it. The man who believes he will be Britain's next prime minister told me the referendum was now his No 1 priority and later, in the community centre, he acknowledged victory for the No side depended on Labour's performance. Though he managed to avoid saying so explicitly, Labour's performance in the next election is equally dependent on outcome of the referendum. With 41 out of 59 Scottish MPs, it's impossible to imagine Labour winning at Westminster if Scotland voted to leave the UK.

The SNP is no less aware of the importance of Labour's role in the referendum battle. Not only are Labour supporters more likely to be undecided than Tory or LibDem voters, there are a lot more of them. A million Scots backed Labour in the 2010 Westminster election and even when they were thrashed by the SNP in the 2011 Holyrood poll more than 600,000 voted for the party then led by Iain Gray.

But in reaching out to those Labour voters they need to vote Yes - as both Alex Salmond and Nicola Sturgeon did earlier this month - the SNP faces a dilemma. Mr Salmond needs to attack Mr Miliband's party, and with it the kind of policies his own left-of-centre SNP might normally be expected to embrace, because the prospect of a Labour victory next year reduces support for independence, according to polls.

He must do that, though, without alienating the party's supporters. At first it seemed the strategy was for SNP ministers to pull their punches and leave the attacks to independence-supporting commentators who, helpfully, have all arrived at the conclusion that Labour is a dead loss and shared the view with gusto in print and on the airwaves in the days leading up to Mr Miliband's visit. But yesterday that changed, with the First Minister accusing Labour of "abandoning its principles" by fighting the referendum alongside the Tories and LibDems in the Better Together campaign. Mr Miliband, he said, had "zero credibility". It's a sure sign of how fierce the fight will be in those old Labour heartlands.