Six months from today we will witness Super Thursday; the largest test of public opinion this side of the next general election. It will involve the Scottish Parliament, the Welsh Assembly and thousands of council seats in England. And it is already clear that Scotland could be the Government's largest electoral headache. A poll published yesterday showed not only that Labour and the Scottish National Party would win an almost identical number of seats if the contest were held now but that a majority of the electorate north of the Border favours independence. The 300th anniversary of the Act of Union next year does not look as if it will be the cause for much celebration.

A host of caveats should be registered. What this survey reveals principally is the extent of disappointment in what has occurred since devolution and the unpopular character of the Labour-led Scottish Executive. Jack McConnell, the First Minister, has a certain swagger (a sort of Alan Milburn in a kilt) but his achievements of substance are a wee bit modest. His failings have been shrewdly exploited by the SNP, which has been revived by the return of the charismatic Alex Salmond to its leadership. The SNP has, though, endured many false dawns before, only to see votes slip away when people begin to focus on the real costs of its policies. The party will again be placed under serious scrutiny as polling day approaches.

Despite this potential weakness in the SNP, it would be wrong to assume that Scotland will return to electoral business as usual. Labour is almost bound to lose seats, the SNP to advance and the Liberal Democrats to make major progress. The Greens could do well, while both the Conservatives and the hard Left (for rather different reasons) may flounder. Virtually every analyst expects that the Liberal Democrats will be cast as kingmakers. Would they continue in alliance with Labour or throw in their lot with the SNP despite disagreeing on independence?

This dilemma for the Liberal Democrats if they do “too well” in May is an acute one. Their instinct may well be for Labour, but if the SNP won the most seats it would be awkward to be seen to be denying it the chance to form a government. The Lib Dems’ present line is that they would not be prepared to do business if Mr Salmond insisted on calling a referendum on independence in the next four years. However, that stance may be hard to sustain if electors believe that the SNP has a mandate for such a ballot.

The end result will have profound implications for the rest of the United Kingdom. Devolution has never had to be tested with an administration that opposes the ruling party. The relationship would plainly be more antagonistic, but no one really knows what the rules of this new game would be. Devolution has not caused major political conflict so far, in large part because of the deference shown by local Labour leaders to their national counterparts.

It would be especially paradoxical if, at a time when Britain acquired a Scottish Labour prime minister in Gordon Brown, Scotland discarded a Labour first minister. Indeed, Labour’s best hope may be that a “Brown bounce” persuades the Scots that devolution will work better for them with him at the helm despite their reservations about Mr McConnell. If not, the country and the constitution may face turbulent times.