If the Conservatives really think that "Vote Ed get Nicola" (or even "get Alex") will win them the election then they are strategically naïve. Of course this is possibly true - after all with the same strategist they imagined "Vote Blair get Brown" would be a vote-winner in 2005 and we all know how that went.
But actually, the Tory line on the SNP is not so much about getting votes now as about controlling the post-election scenario. As such, with this now being the only focus of Tory campaigning, they appear to have accepted that they are not going to win a majority. Not only that, they are acutely aware that there is unlikely to be available arithmetic for them to find a majority. People talk about Liberal Democrats, DUP and UKIP but even if the numbers are there (and the models don't suggest they would be) even the Liberal Democrats might balk at a deal with UKIP and, more importantly, their prices are likely to be entirely contradictory. The Lib Dems would want to lose the EU referendum; UKIP and the DUP would demand it as soon as possible. Only a few weeks ago, the Tories appeared confident (I felt surprisingly so) that they would win quite comfortably in the end; it is now increasingly difficult to see a scenario where Cameron could lead a functional government.
So what's the strategy? The Tories have accepted that they aren't going to win a majority, but they want to be certain that Labour won't win one either. As such, it is (secretly-ish) in Tory interests for the SNP to conquer all in Scottish seats. Their "spinners" have talked up Nicola Sturgeon after both debates, and they are pushing Labour into making anti-SNP comments which is doing Labour no favours in Scotland. They have also successfully got Ed Miliband to rule out a coalition with the SNP and are doing everything they can to get him to rule out any other sort of arrangement too. That way, even if Labour are the biggest party, if the arithmetic is not there to get a majority with the Liberal Democrats and the SDLP, Miliband will be left with very few options. It almost seems like the Conservatives, who 5 years ago created fixed-term parliaments, might be putting their hopes on a second general election!
It's a clever strategy in some ways and one which Labour needs to counter. But actually Labour should feel empowered by this: the Conservatives are on the back foot and, as the odds suggest, people who bet on Ed Miliband to be the next Prime Minister a few weeks ago should be feeling quite confident now.