A party like no other

It is important not to misread the mood of Scotland as it goes to the polls tomorrow. The desire to punish Labour is palpable, as it is throughout the UK. The difference is that in Scotland the Conservatives remain a broken force and the SNP is the only credible alternative. There is no evidence that this equates with widespread support for the SNP's flagship policy of separatism - those wanting to break up the UK account for at most a quarter of the electorate, according to the polls. (Higher numbers in favour of "independence" include many who want more devolved powers within the UK.)

Whatever the headlines say, tomorrow's vote will not signal Scotland's rejection of the union, as even the SNP has acknowledged. In order to broaden its appeal and take the constitutional issue out of play, it has emphasised its pledge to put the issue to a referendum. The message is one of reassurance: unionist voters angry with Labour, or weary of its stranglehold on public life, can indulge their desire for change without imperilling the union. Or can they?

The essential point about the SNP is that it is a party unlike any other. While its rivals develop programmes around socio-economic choices, the SNP is at root a single-issue party, devoted to establishing an independent state. That is why it has been so ideologically footloose over the years, positioning itself as the "Tartan Tories" in the 60s and more socialist than Labour in the 80s. Now it offers something for everyone, with a Thatcherite economic platform allied to a leftist foreign policy.

Those voting SNP should therefore be prepared for the consequences. An SNP executive would take every policy decision with the intention of undermining the union. The aim would be to manufacture consent ahead of a referendum in 2010, by provoking a series of crises that illustrate the supposed bankruptcy of the devolution settlement.

Trotskyites called this the tactic of the "transitional demand". The idea is to advance reforms that are superficially reasonable but inherently unworkable, leading to frustration and conflict, and support for more radical change. Indeed, the SNP's programme for the first hundred days contains a number of classic transitional demands, from the repatriation of oil revenues and Scotland leading EU fisheries negotiations on behalf of the UK, to a Scottish Olympic team.

This is partly aimed at the people of Scotland, distrusted by the SNP as "90-minute patriots" for expressing national identity largely through sport. But the bigger prize is to provoke an English backlash that would force Scotland's exit from the UK. Alex Salmond can barely contain his excitement at polls suggesting that support for dissolving the union is higher in England than Scotland. It is a bizarre paradox, but middle England is now the SNP's real revolutionary vanguard.

The SNP's quest for independence would lead to economic and social choices that are very different from the ones Scottish voters have traditionally supported. That is the source of its appeal to business leaders and former Conservatives. Their analysis mirrors the "subsidy junkie" jibes of the Daily Mail in its insistence that separatism will end Scotland's "dependency culture" and force a leaner, meaner, free-market future. Even the Adam Smith Institute has endorsed the separatist agenda.

Although analysts estimate that high oil prices mean that Scotland is in a good financial position, declining prices and production are likely to mean that an independent Scotland would have to choose tax rises or spending cuts within a decade. The SNP has anticipated that shift by adopting a Thatcherite small government, low-tax populism. The desire for change is understandable. But a protest vote tomorrow will be no free hit.

David Clark, a former Labour government adviser.