There is some evidence that the Conservatives have finally found a scare story that works. We probably will not know how large or long lasting it will be until after the election. However, as scare stories are generally myths, and I now have a professional interest in mythmaking, I thought it would be worth asking why this one has stuck whereas earlier attempts have failed. 
Here were some earlier but unsuccessful attempts.
1) ‘Labour will bankrupt the economy, again’. Given mediamacro, this should have worked. But I suspect this line was ruined when Cameron started to promise to tax less and spend more and reduce the deficit. You cannot base your fiscal policy on home economics and then ignore the household accounts.
2) ‘Labour will put up your taxes’ flopped, perhaps because voters didn’t mind too much if this helps save the NHS. A smaller state is just not popular, which is probably one reason why they had to make so much of deficit reduction.
3) The ‘Miliband looks funny’ strategy fell apart when people realised he was rather better than much of the press made out. The problem here was that there was no half-truth to build a myth upon (beside a rather dark one), but the Conservatives believed their own propaganda.
So why has the Lab+SNP=chaos line worked? A myth it certainly is. If you want chaos, see what will happen to the Conservative party during the EU referendum. One scenario I have not seen discussed is that a new Con/LibDem coalition breaks apart after the referendum, either because Cameron fails to recommend staying in, and/or because large numbers of Conservative MPs defect to UKIP, which makes the coalition dependent on their support.
With my mediamacro experience, I can think of three reasons why this myth has stuck. First a successful myth has to be based on a half-truth, and the half-truth is that the SNP would have some influence on any Labour government. Not much, because to vote down a Labour government would be a huge gamble for the SNP. Their support in Scotland could disappear overnight if they could be charged with letting a Conservative government back in without due cause. But clearly there would be some influence, which is only right in a democracy.
Second, the non-partisan media finds it difficult to counter a myth when no major political party is calling it a myth, particularly during an election. The SNP have encouraged the myth: some would unkindly say because they want a Conservative government, but even if that is not true they want to talk up the influence they would have on Labour. Labour itself does not want to promote the idea that they could happily work with the SNP because they in turn want to scare former Labour Scottish voters from voting SNP. With no political party challenging the chaos myth, the media finds it very difficult to do so off its own bat. A few journalists like Philip Stephens in the FT can add some reality, but if politicians are not being challenged repeatedly on the news, then there is little to counter the formidable power of the right wing press.
Third, this is new territory, with few reference points, so people cannot use their own experience of similar situations in the past. The parallel with austerity would be the Eurozone crisis.
But before I convince myself, there may be something less myth like and more basic going on here: pure and simple nationalism. Although many on the left would like to believe that the Scottish independence referendum marked a new engagement with politics away from the Westminster elite, it could also just be another example of the political power of nationalism. And if nationalism can have so much force north of the border, it is not surprising that there should not be at least some echo of this in England. English feelings of resentment and unfairness might be perfectly justified, but their monetary and political importance is tiny compared to the huge differences between the political parties on other issues. But nationalism does not respect that kind of calculus.
So maybe this all has nothing to do with chaos theory, but is simply about a more basic strategy: divide and rule.
 There has been some criticism within the Conservative party about the negative character of their campaign. Why not focus on the positive achievements of the last five years? What is not clear to me is whether this was ever a viable strategy. In my own sphere I can think of one positive achievement, which was setting up the OBR, but I suspect I attach more weight to that than the average voter.