Michael Moore's sacking was a surprise insofar as many reckon he has done a decent job as Secretary of State for Scotland.

His unflashy style may not have made many waves, but he is measured and calm, and his impact has been significant.But the first explanation to emerge for his return to the back benches is the most likely.

Mr Moore has been replaced because Nick Clegg feels he needs more of a political bruiser at the helm at the Scotland Office, ahead of the independence referendum. With less than a year to go, Alistair Carmichael's set of skills seems better suited.The calculation appears to be that his lawyerly eye for detail, combined with a more ready wit and a degree of charm, will better serve the pro-Union campaign in the final months than Mr Moore's somewhat stiffer style.

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As with everything in this debate, there are other interpretations, of course. The SNP claim Mr Carmichael's role will be to spread 'scare stories' in line with the perceived negativity of the Unionist campaign. It has also been suggested by the pro-independence camps that the change is indicative of panic in the cross-party pro-Union campaign.

Is it? Not really. The polls show a healthy lead for the pro-Union camp. There is no evidence they are either panicking or need to.

The reason for the sacking is more straightforward. Mr Moore's style, calm, measured and balanced, was suited to navigating the 2010 Scotland Act through parliament. Containing much compromise, which as a result pleased nobody in particular, the act was nevertheless a complex piece of legislation and Mr Moore deserves credit for his leading role.

Similarly, he deserves praise for helping to negotiate the Edinburgh Agreement that paved the way for the referendum. Even his opponents acknowledge the importance of his constructive approach.

Now, with battle lines drawn in the independence debate and time running out, Mr Carmichael may be a more suitable figurehead for the Unionist cause.

The SNP say the problem for the No campaign is not the messenger. And it is not. But, in a TV debate in the May, Mr Moore came off worst when pitted against the Deputy First Minister. Labour's Anas Sarwar arguably lost a bad-tempered TV encounter with Nicola Sturgeon in September.

Mr Carmichael might be expected to put up a better fight against Ms Sturgeon in such a setting. Indeed his experience in doing so when he was Shadow Scottish Secretary may well have contributed to his elevation. What will he bring to the debate? Perhaps something more positive than the Yes campaign predicts, although it will seek to capitalise on concerns that a negative bruising battle is ahead after Jim Murphy, the demoted former Shadow Defence Secretary (see below), said he looked forward to playing a bigger role in Better Together. With regard to Mr Carmichael, there may be more humour than from Mr Moore, whose 'Mr Reasonable' approach could leave him looking defensive. Either way, Mr Carmichael's promotion increases the chances of a more engaging, debate. How will he gel with the abrasive Mr Murphy?