It needed to be good.
It needed to boost Better Together's troops after Monday's referendum debate between Alistair Darling and Alex Salmond, which was won by the First Minister. At the very least it needed to avoid controversy. David Cameron's speech at the CBI Scotland dinner last night did in fact make all the right noises in that regard, urging voters to choose "openness over narrowness" and the great advantages of the UK over the great unknown. But his message was still knocked comprehensively off course. After the Prime Minister spoke of stability as one of the great benefits to Scotland of remaining in the UK, he found himself rebuked by Sir Mike Rake, the CBI Scotland president, over the instability caused by his very own promise of an in-out referendum on EU membership. Ouch.
It is not clear whether or not the chairman of BT Group intended to overshadow Mr Cameron's speech with his remarks. He is, after all, a strong proponent of Scotland remaining in the UK. But that was the effect. Talking of the "real uncertainty" that would be caused for business by a vote for Scottish independence, he added that the proposed EU vote in 2017, along with the General Election next year, "contribute their own specific share of uncertainty". Membership of the EU is crucial to jobs and growth, he went on, but the ambiguity caused by the proposed in-out EU referendum was "increasingly causing real concern for businesses regarding their future investment plans".
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He is, of course, entirely correct: the spectre of an in-out EU referendum does indeed cause uncertainty, while the prospect of leaving the EU would be drastic for Britain.
When it comes to EU membership, there is uncertainty, of course, whichever way the vote goes on September 18. Opinion is sharply divided over whether an independent Scotland would have to reapply to join and how long admission might take, not to mention whether it would achieve the favourable terms the UK has negotiated.
Still, that point will have been lost in the face of Mr Cameron's embarrassment. The focus is now on his promise of a referendum on EU membership and the English Tory Party's Ukip problem, both of which have been potential pitfalls for the Scottish pro-UK campaign all along.
Yesterday also marked the day that Conservative MP Douglas Carswell defected to Ukip, highlighting the pressure that the Conservative party in England is under from Eurosceptic elements.
MPs do, of course, cross the floor from time to time without long-term consequences for their parties. In spite of Ukip's successes in the European and English local elections, their views do not reflect those of the majority of English voters (here in Scotland too, it must be remembered, voters elected a Ukip MEP). Even so, it does the Better Together campaign no favours to have this issue erupt now, reminding many Scottish voters of what they deplore about certain aspects of politics elsewhere in the UK.
The pro-UK campaign must rally if it is to regain the initiative in time for polling day.