Cameron's motive is clear - he wants to prevent Miliband from forming a government with SNP support (or in any other way, of course). But what is Miliband playing at?
We all know that Labour hates the SNP for stealing its votes in Scotland. The imminent loss of so many Scottish seats will prevent Labour from winning outright - possibly from winning at all. So we can hardly expect goodwill. But why should a desire for independence put the SNP beyond the political pale?
Independence is a legitimate political desire. It's one we often support, at least abroad, and sufficiently legitimate that Scotland was allowed a referendum to decide if it wanted it. It's not like support for terrorism or torture or racism - positions that ought to make collaboration unthinkable.
So if the election gives the SNP 40 seats it may well have more than the LibDems. It will be the third largest UK parliamentary party with a just claim to be considered as a coalition partner.
So what Miliband should be saying to the SNP NOW is.
We agree with you on many issues. We disagree with you on issues like Trident and independence but if you become a major UK party and Labour is the largest party we will talk to you. You mustn't expect to get independence that way; remember you lost the referendum! But politics is the art of the possible and if we can't defeat you we'll just have to negotiate.That's not just necessary and inevitable, it's also right. If the SNP wins 40 seats it will have a mandate to represent those voters. Miliband should swallow his pride and listen.