IN a speech in London today, Alex Salmond will present his case for Scottish independence in considerable detail.

He will explain why he believes an independent Scotland could and should share the pound in a currency union with the rest of the UK.

He will set out why he thinks an independent Scotland would promptly and seamlessly take its place among the member states of the European Union.

But he will go further. In parallel with his well-rehearsed economic arguments, the First Minister will make a much more emotional pitch to voters than we are used to hearing. "It's Scotland's hour," he will say of the moment when polls open on September 18. On polling day, the people "will be truly sovereign", he will argue. His message is straightforward: this is a historic opportunity not to be missed.

His words echo almost exactly remarks by Deputy First Minister in a speech in Glasgow yesterday. "We will be a sovereign people," she said, "for just a few short hours." What will you tell your grandchildren, she asked, if you hand control of your destiny right back to Westminster?

Constitutional lawyers will doubtless raise an eyebrow or two at the notion of temporary, polling day sovereignty. But the impression it creates is a powerful one and it is hard not see Mr Salmond's hero, Robert Burns, guiding their rhetoric. "Now's the day and now's the hour," as the Bard wrote, imagining Bannockburn, in Scots Wha Hae.

This striking new appeal to the heart by both the First Minister and Deputy First Minister, the two politicians driving the independence campaign, is no accident. It marks a new phase in the fight. Up to now, the independence debate has been largely technocratic. When Mark Carney, the Governor of the Bank of England, intervened to spell out the limitations of monetary unions, as he sees them, the printed text of his speech included a series of graphs.

It seemed to sum up a debate that, even when it has produced heated arguments, has been dominated by cold, hard issues. Of course, the possible currency of an independent Scotland is of fundamental importance.

Likewise, an independent Scotland's membership of the EU, its armed forces, its pensions and benefits system and its economic prospects all deserve the most detailed scrutiny possible. But both sides must seek to win hearts as well as minds. They must not only reassure but inspire.

Mr Salmond's appeal to a sense of history, a sense of moment, is part of an attempt to do exactly that. It comes as the latest poll suggests uncertainty about the currency is beginning to nudge undecided voters towards the No camp but it is not a reaction to that. His theme will develop as polling day draws closer.

For the past 18 months the battle over Scotland's future has been fought between Nationalists wary of appearing too Braveheart and Unionists reluctant to wrap themselves in the Union Flag.

They have been absolutely right to put policy before emotion. But, with fewer than 200 days to go, the prospect of an appeal to the heart as well as the head seems as timely as it is welcome.