John Curtice, widely acknowledged to be Scotland's only living psephologist (if another one is ever discovered it'll be a bigger story than the comet landing), has made a superficially fair criticism of the SNP's strategy for a hung parliament at Westminster. During the BBC coverage of the party's conference, he suggested that they were making a mistake by completely ruling out a deal with the Tories, because that will reduce their leverage over Labour in any post-election negotiations (for example, witness the way in which the Tories only upped their offer in 2010 after the Lib Dems entered into talks with Labour).
There's a big flaw in Curtice's line of thinking, though, which is that he's looking one tactical step too far ahead. There's not much use in having the perfect strategy for what to do when you hold the balance of power if you never get to that point in the first place. And even leaving open the slimmest of theoretical possibilities that the SNP might deal with the Tories would only hamper the chances of a significant breakthrough next year. Voters need absolute clarity that putting their faith in the SNP will not let the Tories in by the back door - and Nicola Sturgeon has just given them that clarity.
In any case, the leverage a large SNP bloc could have over Labour in a hung parliament, even without the threat of putting the Tories in, should not be underestimated. It's not true that Labour could say to Nicola Sturgeon "you have nowhere else to go", because she would always have the option of walking away and letting Labour try to govern through ad hoc deals with UKIP and the DUP. A few months of that might just bring Miliband and Balls to their senses.
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A couple of hours ago, the Lib Dem blogger Caron Lindsay said on Twitter that a "one-party state" was unhealthy, and that greater plurality and diversity was needed. She didn't elaborate on what she meant, but I can only assume that Scotland is supposed to be the "state" and the SNP is supposed to be the "one-party". Well, that's curious, because of course Scotland is not a state (it's a distinct legal jurisdiction, but that's not the same thing). The head of government in the state we inhabit is not Alex Salmond or Nicola Sturgeon, but David Cameron - and it's scarcely people on our side of the argument who are responsible for putting him there and keeping him there.