Of course it was opportunism - that's what politics is most of the time, and it is practised by politicians on all sides of the debate. However, that doesn't invalidate the argument.
As long as Cameron plays his cards carefully, he will control this debate. The Labour Party's argument that the Scottish MPs are "United Kingdom" MPs falls flat, as they are easily open to the accusation that the great majority of those MPs are theirs - without them they will struggle to hold a...