As the politician who led the Better Together campaign during the Scottish independence referendum, the former Chancellor is better placed than most to navigate the potential political pitfalls of discussing issues of nationality.
Westminster may suspect, often correctly, that calls by the SNP for greater powers for Holyrood are simply mischief-making – designed to impress the supporters and shift blame for problems – or efforts to create another stepping stone towards a separate state.
But, particularly given the electoral dog-fight between Labour and the SNP, Mr Darling’s intervention adds weight to the idea that it is one of the few possible solutions to a looming demographic timebomb.
Recent figures show Scotland’s population is set to increase, but this is largely driven by inward migration.
If this supply of largely young, energetic people is even partially cut off, Scotland’s economy may start to suffer. As our population ages, will a dwindling workforce struggle to pay the taxes that fund the required level of social services?
The practical difficulties of setting up a separate immigration system could prove insurmountable. And the Westminster Government has a considerable job to do in negotiating the Brexit deal with Brussels so the civil service may simply not have the time to look into it.
But if Westminster disregards Mr Darling’s advice and neglects a warning about what appears to be a genuine problem, it may find the level of discontent with Brexit grows even higher in Scotland, where 62 per cent of the population voted to remain in the European Union in last year’s referendum.
And that discontent could be turned into support for the idea that Scotland would be better off as an independent country within the EU, rather than part of post-Brexit Britain.
So creating a separate immigration system – handing a significant power normally exercised on a state level – may in fact help keep the UK together.