Bruce Anderson: David Cameron's speech yesterday was the best he has yet delivered. The Unionist fightback has begun.

Yesterday, David Cameron gave the best speech that he has yet delivered. It was in Scotland, in defence of the Union. Its quality rested on its strengths: rhetoric as a fusion of emotion and intellect. In proclaiming the transcendent importance of the Union, in committing himself to battle in the fight for the future of his country, the Prime Minister drew on deep feelings, passion and idealism. The conviction and the sincerity were heart-felt. Hardened hacks of my acquaintance, who have heard a speech or two in their time, were moved and misty-eyed. Suddenly, the beleaguered pro-Union forces have heard the skirl of the pipes and are ready to rise up and march and fight and roar.

The idealism was palpable. At present, the Tories have one seat in Scotland. Mr Cameron, who can add one and nought, realises that it does not equal 30 (half the Scottish seats plus one). As long as Scotland is in the United Kingdom, it is harder for the Tories to win outright. Given the Conservative party's reputation as the proponent of a hard-faced, counting-house, anti-sentimental approach to politics, one might have thought that this calculation would be all that mattered to the average English Tory. Not a bit of it. In standing by the Union, even in adversity, in defiance of any narrow view of self-interest, English Toryism is vindicating its claim to be the true National party. I cannot commend this speech highly enough. Everyone should read it. It should renew and refresh every Tory's pride in his creed. It is an historic document.

But it is only the beginning. There is a deformation professionnelle which afflicts all politicians. They make a speech. Everyone applauds. So they assume that everyone will remember everything that they said. To put it mildly, that is over-optimistic. There is wisdom in the old adage. Tell people what you are going to say. Say it. Then tell them that you have said it. The Blairites understood the value of repetition, partly because they rarely had anything new to say. But any sensible politician ought to be willing to repeat himself until he is sick of his own words. Then, suddenly, someone will pay attention. "That's a good point. Why haven't you made it before?" At that stage, the politician must control his natural desire to explode with incredulity and wrath, consoling himself that the message is at last getting across.

Yesterday, it did. But there is a danger. Mr Cameron, who does not lack self-confidence, might conclude that if ever the Union cause were in trouble, he would just have to go to Scotland and make a speech. That is not enough. We need a sustained pro-Union campaign, drawing on all the Unionist parties and on the many Scots who have dropped out of politics because they have despaired of finding anything worthy of their adherence. Alistair Darling could play a crucial role. After long years in government, he has matured into an elder statesman. He also possesses a deal of Scottish cautious wisdom.

There are two Scottish political temperaments. There are the perennial Jacobites, who would hazard everything as they took to the hill and the heather in a desperate struggle for a doomed kingdom. There are also the pawky realists, the Platonic idea of a bank manager: over the past few years, the once-great Scottish banks could have done with a few of those. Scott and Buchan, who both loved their native land, recognised the two archetypes, created characters who exemplified them, and then brought them together in a romantic harmony: easier, perhaps, in fiction than in life. As befits a former Chancellor, Mr Darling is on the pawky side of the ledger. He brings to public debate some of the chill of an old-fashioned bank manager from the days before call centres, subjecting an anxious customer to an awkward interview about a burgeoning overdraft. When Scotland needs a cold shower, when the debate would benefit from a stiff walk along George Street, Edinburgh, in the biting east wind of a clear winter day, Alistair Darling is the man.

There are plenty of romantics, including John Reid, another former Labour Cabinet member who will be as passionate as the PM in the defence of the Union. Over the past few months, there have been times when Scottish Unionists have come close to despair. Nothing seemed to be happening. Alex Salmond was scoring at will, all around the wicket, making a monkey of the other parties. The Union, that greatest of constitutional achievements, seemed to be slipping into a coma. No longer: the fight-back has begun. That will need a lot more than one speech. But at least the speech has been made. "I'm ready for the fight for our country's life", the Prime Minister said yesterday. That is a fight which must be won.