I had originally drafted a large part of this post a few weeks ago, and had forgotten about it. I thought it might be apposite to put it up on the day of the TV debate.
Politics in the UK is in the full campaign mode. The outcome is very difficult to predict this time, and given there is plenty of time for someone or a party to make a bad gaffe or mistake, only a clairvoyant or a fool would foretell the results. One big factor making any projections well-nigh impossible is the ascendancy of smaller and national parties, such as UKIP, the Greens, and the SNP. The margins of victory might be very small depending on the circumstances and dynamics of each constituency, and few votes going one way or another could make a difference, and it is not simply a switch between the established Westminster parties: Conservative, Labour, and Liberal Democrat.
Labour is putting forward an argument that voting for the SNP is akin to voting for Mr Cameron. This is absurd. It can only be true, if the anti-Tory vote is split between Labour and the SNP and that lets the Conservative candidate win the seat. I am not a clairvoyant and hopefully not a fool, but I am fairly certain that the Tories are not going to win many seats in Scotland. What Labour is saying is that a Labour-majority government becomes less likely should voters in Scotland return SNP MPs to Westminster.
I hazard a guess that Scots are very aware that the composition of the next UK government will be decided primarily in England. Given the West Lothian question, which can only become more pronounced with further devolution of powers to Holyrood, the next UK government will in all likelihood have to command a majority among MPs from constitutencies in England. Whatever happens in Scotland, for Labour to form the next government on its own or in a coalition, it has to win more seats than the Conservatives in England at the minimum, and preferably a majority. Put it another way, Scotland can return many Labour MPs, but if the Conservatives were to win more seats than Labour in England, then a Labour-majority or Labour-led government is very unlikely. Given that, Scots voting for the SNP are voting for the SNP, which might or might be part of or support a Labour-led government.
Voters are fully capable to cast their ballots strategically to keep one party out. While scare or negative tactics might work, it does seem a little peculiar that the central narrative from Labour to Scottish voters is not the desirability of a Labour-majority government, but somewhat illogical argument that voting for the SNP is voting for the Conservatives. Perhaps Labour strategists think this is an effective method, otherwise an alternative explanation may be that Labour does not have a clear message for Scottish voters.