Scottish Independence: David Cameron Pleads for Scots to Not Undermine and Diminish Britain
Cameron has visited Scotland in independence campaign Reuters

David Cameron knows the anti-independence campaign in Scotland will get nowhere by focusing on Tory support, because there isn't any worth mentioning.

So he has kicked off a two-day visit to Scotland with an audacious appeal to Labour sentiments instead.

In a surprising pitch for a "no" vote, the prime minister invoked the memory of Labour's former leader and architect of Scottish devolution, John Smith, who died of a heart attack in 1994.

"Twenty years ago this week, the Labour leader John Smith died. Whatever people thought of his policies, nobody could argue that he was a proud Scot who wanted the best for his country," he declared.

"And why not? Like millions of other people, he knew that loving your country and at the same time wanting to be part of something bigger does not make you any less Scottish.

"That truth is shared by millions of others. So my message is simple. We want Scotland to stay. We are all enriched by being together. Scotland puts the great into Great Britain. Together we are a United Kingdom with a united future."

The prime minister's latest visit to Scotland comes after reports the Better Together campaign had sidelined its Labour head, Alistair Darling, in panic over the narrowing opinion polls.

The suggestion was pretty swiftly and comprehensively slapped down by Downing Street and was seen as part of the briefings which have recently been aimed at undermining the campaign.

It allowed deputy first minister, Nicola Sturgeon, to claim the "no" campaign had now been taken over by the Tory party, a suggestion that would only serve to damage it.

And first minister Alex Salmond once again repeated his demand for Cameron to meet him in a face-to-face TV debate over independence, which the prime minister has flatly ruled out on the grounds such a debate should be between the leaders of the two opposing camps.

So far, however, there has been no sign of such a debate between Salmond and Darling.

Cameron knows he has to tread carefully in this latest visit and the wider Unionist campaign because of the deep ill-feeling towards the Conservatives in Scotland which dates back to, at least, Margaret Thatcher's time.

If he is seen to be avoiding Scotland he will be accused of running scared in the face of the Salmond challenge, while he also knows the resentment felt towards the Tories makes it hugely difficult for him to be a cheer leader for the no campaign.

Meanwhile his party in Westminster need to see him doing whatever he can to bolster the Unionist cause.

So his carefully choreographed visit aims at walking that difficult tightrope.