With the launch of the SNP manifesto, the four main contenders for power at Holyrood have presented their proposals to the electorate and, as SNP leader Alex Salmond put it, await their instructions from the Scottish people. Promises yesterday ranged from scrapping business rates for 120,000 small businesses, increasing free nursery places by 50%, abolishing bridge tolls, cutting primary school class sizes to 18 to removing the graduate endowment tax as well, of course, as replacing council tax with a local income tax. It was all delivered in impressively professional style and the adjective "statesmanlike" was even heard among the press corps.

With a broad similarity between many of the manifesto commitments on the day-to-day issues of education, health, justice and the environment, the biggest differentiating factor between the major parties is the degree of devolution they espouse. Yesterday the prospect was raised of a second referendum on independence if the prospect were to be rejected in 2010 but the SNP subsequently returned to power. The possibility of this "neverendum" brings the charge that continuous uncertainty over the constitutional issues will siphon political energy away from more immediate policy-making as we continue to wrangle over the constitutional issues that have defined Scottish politics since Winnie Ewing won the Hamilton by-election for the SNP exactly 40 years ago.

As the party of independence, the SNP must, if it holds power in Edinburgh, deliver the referendum it has promised. Yet it and the voters are all too aware that many people who seem poised to vote SNP in 2007 are unlikely to support full independence in 2010, if the evidence of the polls is accurate. There is a further understanding that a likely outcome on May 3 is a coalition administration and that, again on the evidence of the polls, the more probable partners are the SNP and the LibDems. That combination, along with other possible permutations, including the status quo or a minority Labour administration, will all be in the minds of voters recording their constituency and list preferences. Inevitably, this provoked questions about a coalition agreement and it was significant that Mr Salmond, when pressed on the prospect of including a question on more powers for Holyrood in an independence referendum, refused to rule it out.

Since that chimes with the policy of the Liberal Democrats, who yesterday ruled out agreeing to a referendum in order to form a coalition with the SNP, it would suggest both parties are privately considering how far they could go to overcome their differences. That, in turn, must focus attention on the common aims, in particular, the replacement of the council tax by a local income tax. Where the LibDems want a locally variable rate of between 3.5p and 3.75p in the pound, the SNP proposes a flat rate of 3p in the pound to achieve what Mr Salmond calls "the biggest tax cut in a generation"; but, more important, is exactly how that will be implemented to fund fully local government services. That will be the test of the SNP's credibility as a party of government rather than opposition, including whether it could negotiate to receive the money Scotland currently is awarded from the Treasury for council tax benefit.

How that relationship is maintained will be the defining feature of any SNP-led executive: while insisting on the referendum, Mr Salmond was equally emphatic that England would be the "biggest pal" of an independent Scotland. That must be non-negotiable.