WHY is David Cameron so keen to have a fight with Alex Salmond, a much more heavyweight and credible opponent than Ed Miliband will ever be, on the timing and form of the forthcoming referendum on Scottish independence?

I reckon the Prime Minister is posturing to appease his backbenchers. These backbenchers are truculent and mutinous. The Tory whips already, after less than two years, despair of them. Most are new to the Commons but it has not taken them long to assert themselves and many have been relaxed about voting against their own coalition.

They are not disposed to be in any way deferential. Some of them bitterly blame Mr Cameron for not winning an overall majority in a general election they regarded as eminently winnable. In that sense, they see the Prime Minister as a failure –and they don't like being bossed around by a man they see as weak. Many of them intend to make more trouble over Europe.

This difficult political landscape is made even more problematic for David Cameron because he will have to undertake a major reshuffle before long; he cannot put this off for much longer. Normally this would be an opportunity to use his patronage to get some of the leading troublemakers into government – in effect to shut them up – by the simple expedient of giving them junior ministerial office.

And Mr Cameron has another problem here – he will have considerably less patronage than a premier normally would, because he leads a coalition, and some promotions must be found for Liberal Democrats. Further, some of his dissident backbenchers will be annoyed, and potentially even more mutinous, if they are passed over.

As he scans this darkening landscape David Cameron –who I think is a genuine Unionist – must be aware that tactically, Scotland is already lost ground. There is going to be no Tory revival in Scotland in the foreseeable future. So it is not necessarily bad politics, in a tactical sense, for Mr Cameron to pick a fight with Alex Salmond on a constitutional issue. This will resonate badly in Scotland but it might go down very well south of the Border. In other words, David Cameron needs a populist, rabble- rousing issue to pacify his backbenchers, and maybe he has found it.

But then another problem arises for Mr Cameron. It's all very well to start a fight, but what happens if he does not win it? He may be prepared to antagonise Scottish voters – and not just SNP supporters – but what if he loses to Alex Salmond in the coming battle? The constitutional niceties are not clear cut. It could be that most legal experts think that overall the UK Government has ultimate sovereignty, and thus the UK Government can overrule Holyrood.

That might – possibly – be the ultimate constitutional position, but it is rotten politics. It is rotten politics because Alex Salmond leads a majority government, despite a voting system that was specifically designed by the late Donald Dewar and others to prevent overall majorities. This gives Alex Salmond colossal political clout, which he won't be slow to deploy. Whereas Mr Cameron has no overall majority and many of his backbenchers think he failed because he did not achieve one. The backbenchers loathe coalition government and the man they blame is their own leader. Thus, in terms of the brutal realities of political strength, as opposed to constitutional small print, Mr Cameron is in by far the weaker position.

And the constitutional aspect is fraught as well as complex. As the entire world must know by now, this is the year of our Queen's diamond jubilee. Serious UK constitutional imbroglios have a nasty habit of sooner or later involving the monarch, however indirectly. That will be the last thing the Queen wants in this year of all years. She would hate to be the monarch of a disunited kingdom, one that is utterly divided as Messrs Cameron and Salmond slug it out.