THE West Lothian Question attends, in deed overshadows, all discussion and debate about Scottish devolution. It goes to the core of the possible conflict between a tax-raising parliament in Edinburgh and a sovereign parliament at Westminster.
It also raises fundamental issues about the rights and entitlements of English as well as Scottish voters. Many, pro and anti-devolutionists alike, consider it the question on which the whole project turns. And it is a question to which Mr Tony Blair has not yet found or framed a convincing answer.
For convenience, readers might think of it also as the West Belfast, West Tyrone, East Antrim or Lagan Valley question. The Stormont experience provides a precedent of sorts. And the on-going Scottish debate finds a resonance in the disposition of both Labour and the Conservatives to see a devolved assembly as an essential ingredient of any new settlement on the North.
Under John Smith and Neil Kinnock, Labour refrained from any cross-reference between Scotland and Northern Ireland. Kevin McNamara had successfully established the North as a place apart. But Mr Blair, and his Scottish spokesman George Robertson, have shown no such hesitation. Both routinely point to the devolution component of the joint Framework Documents to rebut Mr Major's contention that devolution is, by definition, anti-Union.
Mr Major counters that Labour is not comparing like with like, that the proposed assembly in the North is effectively designed to fill the local government gap. If that is so, it begs questions about the entire Anglo-Irish design. For a souped-up county council at Stormont seems an unlikely partner for a sovereign Irish government in the engine room of a dynamic new North-South relationship.
Indeed, the severe limits Mr Major anticipates on any powers to be devolved to Belfast suggests the London-Dublin axis will remain paramount (for all the emotional and symbolic significance that nationalists would attach to the North-South dimension).
As mentioned yesterday, Mr Blair may be having second thoughts about his enthusiasm for a powerhouse parliament in Edinburgh. He may even be relieved of his problem if the Scottish electorate, come the referendum, decides it wants the parliament but not the proposed tax-raising powers. But Mr Blair has promised to lead the campaign for a Yes vote on both questions. And his promise is to deliver the Edinburgh parliament within the first year of a Labour government.
Between now and then (assuming he wins) he will be pushed remorselessly to answer the West Lothian Question. It was framed in the late 1970s by the Labour MP Tam Dalyell as he fought a ferocious battle against the Callaghan government's attempt to create Scottish and Welsh assemblies.
Mr Dalyell highlighted the conflict which would arise when English MPs at Westminster discovered they could not vote on issues reserved to the Scottish assembly, for example on health, education and the environment, while Scottish MPs at Westminster remained free to vote on similar issues as they affected the English.
As Mr Major put it last year to the Centre for Policy Studies: "This audience needs little reminding of the intractable problem Labour's plans raise. The West Lothian Question for a start. Simply put, why should Scottish and Welsh MPs be able to vote on English matters, but English MPs unable to vote on Scottish and Welsh matters?"
The second, attendant question is why, in that event, Scotland should continue to be over-represented at Westminster?
The Liberal Democrats accept that there should be fewer Scottish MPs in London after devolution. And polls suggest the public accepts that they should consequently lose their right to vote on matters affecting England. But Labour, so far, will have none of it.
With the lion's share of Scottish representation in London, Labour would lose electorally from any reduction. And Mr Blair initially suggested his proposal to devolve power to the English regions would restore the balance. But the limited and ill-defined nature of that scheme makes this proposition highly unconvincing.
The Stormont precedent might, at first glance, help Labour maintain the muddle. Northern Ireland MPs were allowed to vote on matters affecting England, Scotland and Wales, while they were unable to table questions at Westminster about Northern Ireland issues devolved to the Stormont Parliament. That obviously ended when Stormont was prorogued in 1972. But the shutters came down again during the 1974 power-sharing executive.
Northern Ireland MPs of that era say they observed their own general convention, didn't meddle in matters exclusive to other parts of the UK, and exercised extreme caution in opposing any measure not applying to the North.
But they managed to annoy Harold Wilson in the mid-1960s, when their opposition to a housing Bill prompted him to wonder why he did not enjoy reciprocal rights to interfere in Northern Ireland. Wilson went on to win a comfortable majority in 1966 and the anomaly continued.
Other issues from that time will be reflected, and magnified, in the Scottish debate. Membership of the Westminster club was often regarded as a bit of a sinecure, with the Stormont parliament the focus of local and media interest.
The essential link between the MP and his constituents was weakened, and the additional layer of representation was not always a guarantee of more effective redress of local concerns. Star-quality politicians like Robin Cook will eventually have to decide whether Westminster or Edinburgh represents the premier league, and where they want to play.
On the big issue - how many of them get to play at Westminster - the Stormont precedent lets Labour down. For the 50 years of its existence, the North was under-represented in London. That anomaly, in turn, persisted from 1972 until the 1983 election. In opposition, the newly integrationist Conservative Party will hardly give Mr Blair as long to answer his Scottish question. And we may take it that the threatening giant of English nationalism, so evident in this campaign, will be even more a player in the next.