MAKING a virtue out of necessity, Alex Salmond has decided to take the fight to the Unionist parties on the currency this weekend.

Rather than waver in his demand for a sterling currency union after a Yes vote, the First Minister has dug in deeper, insisting there is no economic or political case for a Plan B.

The anti-independence parties have embarked on a "kamikaze" mission by saying they will veto such a currency arrangement, he says today, writing in the Sunday Herald.

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They are "playing with fire", and will suffer a backlash from voters repelled by the tactics.

In truth, of course, Salmond has little choice.

After making a currency union a central plank of his independence-lite pitch to the electorate, he cannot now advance an alternative.

To change course on such a key issue with only five weeks until polling day would be a disaster for him personally and for the Yes campaign.

He knows this full well, as do the Unionists urging him to make the mistake of a lifetime.

So instead the First Minister fights on, with not a little style, by challenging the No camp to admit the toxic financial consequences for the rest of the UK of blocking a currency union.

So far, the Unionists are holding equally firm.

But the battle is far from over, and Salmond has a habit of grinding opponents into submission.

Ed Miliband may have pledged to include a veto on a currency union in Labour's general election manifesto, but that manifesto will not be written until after the referendum.

If the Treasury executed a quick U-turn after a Yes vote and agreed a currency union, all talk of manifesto promises would be forgotten.

Even if the Unionist parties stuck to their guns, Scotland would continue to use the pound within the UK for at least 18 months after the referendum, allowing time for a Plan B to form.

That would be hugely damaging for Salmond and the SNP but Scotland's economy would not drop off a cliff overnight in March 2016 if the country did move to a new currency.

Like much of the referendum debate, what started as an argument about economics and numbers has reached a stalemate with the two sides insisting they are right and the other side wholly wrong.

So in the end it will come down to trust.

In the absence of a divine arbiter, voters must decide whether they trust what Alex Salmond says more than what Alistair Darling, George Osborne, Ed Balls and Danny Alexander say on the issue.

We know whom we trust.