 TaKE a look at your glass of orange juice/Irn Bru/alka Seltzer/G&amp;T (select your drink of choice for this time of day on a Sunday). Is it half full? Or is it half empty? Your answer to that question may decide who gets to govern Scotland for the next five years. This Holyrood campaign is shaping up into a classic contest between optimism and pessimism, between hope and fear. and this weekend it's the sunny side of the Scottish character that's firmly in the ascendancy.  The SNP lead in our exclusive YouGov poll today is testament to an exemplary, pitch-perfect manifesto launch by one of the most impressive political machines in the UK, never mind just Scotland. There's little to choose between Scottish Labour and the Nats in terms of manifesto commitments, but in tone and feel their campaigns could not be more different.  The political orthodoxy today is that you win elections by being more optimistic than your opponent. Last year in the UK general election, David Cameron painted himself as the candidate who represented "hope, optimism and change". The famous Shepard Fairey poster of Barack Obama's face and the word 'HOPE' has lost none of its hold on the imagination. On becoming Labour leader, Ed Miliband tried to steal the prize from Cameron, saying: "We are the optimists now."  That old smoothie Bill Clinton once spelled out what he called "Clinton's law of politics". He said: "If one candidate is trying to scare you and the other one is trying to get you to think, if one candidate is appealing to your fears and the other one's appealing to your hopes, you'd better vote for the person who wants you to think and hope." (It was a message his wife apparently forgot when she ran for the Democratic presidential nomination some years later, and tried to raise fears about Obama's youth, idealism and inexperience.)  and yet, at the risk of sounding like John Knox with the toothache, this is Scotland. This is the land of Jekyll and Hyde, Private Fraser, the Rev I.M. Jolly and the Caledonian antisyzygy. Yes, we are capable of looking to the sunny uplands and seeing possibility in the world. But let's face it, at other times we can be a nation of grumpy bastards. and at the moment, given the state of the economy, there is much to be grumpy about.  What's more, recent political history shows Scots can be persuaded to vote in accordance with their fears and not their hopes. In Scotland, Clinton's law need not apply. For thirty years up to 2007, Labour beat the SNP in campaign after campaign by playing on voters' fears about independence. and last year in the UK general election, Scottish opinion switched decisively in Labour's favour, largely because of an almost primeval fear of the return of a Tory government. That's why Scottish Labour's manifesto begins with these words from Iain Gray: "Now that the Tories are back we need a government in Scotland that will fight for what really matters."  I thought the party that best tapped into the current despond about wage freezes, job insecurity and youth unemployment would be best placed to win this election. I thought the leader who most convincingly demonstrated that he felt the voters' pain, and promised to do something about it, would win. I still think that's going to be a powerful message come polling day.  But Labour has come up against an SNP campaign of such relentless optimism it's proving well nigh irresistible. Modern, attractive personalities like alan Cumming and Mark Millar coo reassuringly in our ears. There's an SNP gig by Midge Ure at the aBC in Glasgow where you can dance away your recession cares to cheesy Ultravox hits. The socially 