JUST how does David Cameron now get involved in the increasingly passionate referendum campaign?

One of the lines the Yes camp has emphasised repeatedly has been Labour's shared mission to save the Union with the Conservatives; its traditional enemy.

Emphasising the discomfort, Jim Murphy, the Shadow International Development Secretary, despite his deep commitment to save the United Kingdom, vowed not to share a platform with the Tories, whom he came into politics to defeat.

During Monday night's debate, one of Alex Salmond's key barbs was claiming how Alistair Darling was "in bed" with the Tories. And yesterday while riding his victory wave, the First Minister pushed the button yet again. "He is in alliance with the Conservative Party in this campaign; that is what the No campaign is," he declared.

Mr Cameron, "a Unionist head, heart and soul", has pledged to fight to keep Scotland in the UK with "every fibre of my being" and promised to get stuck into the campaign.

But some No campaigners believe the PM to be "toxic" to the bid to keep Scotland in the Union family and would like to see him stay out of the fray as much as possible.

The Tory leader will, of course, be damned by the Yes camp if he does get deeply involved in the final three-week push - portraying him as some kind of colonial viceroy on a stopover - and he will be damned if he does not - branding him feart.

Then there is always the fear that if Mr Cameron breaks cover and goes on a campaign walkabout, he will be kebabed by Nationalist campaigners a la Iain Gray.

But while the PM will be mindful of the potential pitfalls of engagement, he is, where the Union is concerned, a conviction politician; he really does believe in the United Kingdom.

Whatever Mr Cameron achieves in politics, it will be overshadowed if there is a Yes vote on September 18. The MP for Witney will be forever known as the Prime Minister who lost the Union; it's an epitaph he will fight tooth and nail to avoid.