John Curtice: Labour has to be careful in its attacks on the SNP – after all, its hands are not wholly clean

Not all of the last two years has been an easy ride for the SNP. It has had to explain how an independent Scotland would have been able to save the country's ailing banks in the wake of the credit crunch. It has had to backtrack on iconic policies such as a local income tax and scrapping student debt. Its challenge to Labour withered in the face of fierce attacks in the Glenrothes by-election last November.

Even so, the first two years of SNP rule have largely gone down well with the Scottish public. The party has consistently remained ahead of Labour in voting intentions for Scottish parliament elections. The First Minister, Alex Salmond, stands head and shoulders above his rivals in the personal popularity stakes. Key to its success has been its willingness to fight for Scotland's interests, irrespective of what London may think.

But nobody in Mr Salmond's government has ever been in office before May 2007, and while the last two years has given them the opportunity to learn how to deal with the day to day business of Scottish government, nothing has prepared them for the storm of the last week. For the first time, the SNP government is caught in the midst of a full-blown international media, political and diplomatic storm. It now really has to prove that it has the ability to govern.

Moreover, its critics have a damaging line of attack. In letting Mr Megrahi go, they argue, the SNP failed to appreciate where Scotland's interests really lie. Can the SNP, they suggest, really be trusted to run the country?

So the SNP's political opponents hope to inflict two crucial wounds – to portray the SNP as incompetent and failing after all to be the party that can best stand up for Scotland, and they can hope to add weight to their case by passing a parliamentary vote against the Scottish Justice Secretary's decision. But the opposition parties have to be careful not to overplay their hand. After all, the UK Government concluded an agreement with Libya that could have precipitated Mr Megrahi's return. So Labour's hands are not wholly clean.

It is far from clear, too, which way Scottish public opinion will fall. One recent poll found that only 50 per cent believe that Mr Megrahi was guilty and that half think he should be returned to Libya. Certainly there is not the near universal chorus of outrage like that from across the Atlantic.

Certainly while the opposition parties are not in a position to bring Mr Salmond down, if Holyrood passes a vote of no confidence in his government, the Scottish parliament will either have to put Labour into power or face the prospect of an immediate election. Labour can only gain power with the Conservatives' permission and that is unlikely to be forthcoming. Meanwhile, Labour itself can hardly risk the prospect of an early election. For the time being at least Mr Salmond looks likely to survive this storm.

John Curtice is the Professor of Politics at Strathclyde University