It's all posturing. The realpolitik will kick in after the election, unless by some miracle and act of Godly mercy Labour gets an overall majority. Here's how it goes for now:

*The SNP know full well that if they wipe Labour out in Scotland there will be another Cameron government, the worst possible outcome for Scotland, so today Alex has said that he'll bring the Tories down as soon as he can if they win. In other words, vote fearlessly SNP and leave the Tories to us. The only problem with that is that an early second election fought in that scenario would put the Tories in for sure.

*The Tories are being quiet in Scotland because they want the SNP to wipe Labour out. They're scared of a Labour recovery there, so they have to keep bleating on about how Labour will end up consorting with the party that would break up the Union. At the same time, by using that specious argument the Tories have burned what bridges there might have been between them and the SNP.

*Labour can't be seen to be courting the SNP for the reason given above, though they will jump at as close a liaison as possible, even a coalition, if they get half a chance if Ed is the biggest party but with no overall majority.

So they all have to be seen as hating one another's guts. There is no scintilla of cross-party friendship that wouldn't have contumely instantly raining down on it. So they have all got to be seen to be nasty to each other.

My view is that by far the least damaging outcome for everyone would be a coalition between Labour and the SNP, and I wouldn't mind betting that, secretly, most members of both parties would go for that, though they daren't say anything until after polling day. Ah yes, democracy, who needs it...