Philip Stephens in the FT says the idea that a Labour-SNP understanding would amount to Labour being held hostage by the SNP is nonsense. He is of course correct. In a vote on any particular issue, 50 odd SNP MPs could hardly impose their will on 600 MPs from other parties. More interesting is what this line tells us about the media, about the current Conservative Party, and about what the future might hold if they remain in power.
First the media. In my continuing series on mediamacro, I stress that myths are best based on half-truths. Half-truths are the grain of truth on which you can erect a huge lie. With the SNP and Labour, the half-truth is that SNP views on an issue could perhaps weigh a little more heavily on Labour than, say, the views of UKIP, because UKIP will always vote to bring down a minority Labour government, but the SNP will not. That fact will never make Labour go where it does not want to go, but at the margin it could nudge it a bit more in one direction. Conceivably, we might get a bit less austerity, we might treat welfare recipients a bit more humanely - that kind of thing. But would we get some policy that was against the interests of the rest of the union? Of course not. Colin Talbot makes it clear how limited the SNP’s power would in practice be here. 
With mediamacro, you generally need some expertise, or some knowledge of the data, to see that the half-truth is very far from the myth, knowledge political commentators may not have. In the case of ‘SNP blackmail’, political commentators have the required knowledge more than most. So for me the success of the scaremongering about a minority Labour government will be an interesting test: is lack of economic expertise or knowledge important in explaining mediamacro, or is control of the majority of the UK press sufficient. There are signs that the scaremongering is working.
As Lord Forsyth (former Scottish secretary in a Conservative government) said, his own party is putting electoral tactics above a historic commitment to the defence of the UK union. This can hardly come as a surprise. The Scottish independence referendum was a close run thing, so you might expect a party with the integrity of the nation at heart to tread carefully in the subsequent days and months to heal wounds. Instead, Cameron chose in the morning after the vote to attempt to wrong foot Labour on ‘English votes on English issues’, saying: "We have heard the voice of Scotland and now the millions of voices of England must be heard." It was a gift to the SNP.
What does all this tell us about the Conservative Party? Does it tell us that it secretly wants the SNP to get so strong that it could win a future referendum and break up the union? No, what it tells us is that this is a party that is prepared to take large long term risks for minor short term political advantage. As I have suggested on a number of occasions, that seems to be a common pattern in its macroeconomic policy (premature deficit reduction and Help to Buy being two obvious cases).
One of the clearest examples of this is our relationship with Europe. The decision to hold a referendum was taken to appease the right in his own party and potential UKIP voters, even though the uncertainty it creates will damage the economy and even though there is no chance that Cameron will be able to renegotiate to any significant extent. But large sections of what we might call the Establishment seem unperturbed as long as it helps return a Conservative led government. The assumption seems to be that Cameron will be able to sort things out when the time comes, and it will be business as usual. As Polly Toynbee puts it, the view is that “Cameron is “one of us” so he’ll somehow secure an “in” result for his 2017 referendum”
This ignores all the evidence about party before country. A Cameron recommendation to stay in the EU will split his party: after the election a majority of MPs may favour leaving, and a majority of party members already do. In two years time, all the senior figures in the party will be thinking about the elections for Cameron’s replacement. (This is why Cameron’s announcement that he would step down before 2020 was so significant.) In this situation, what are the chances that Cameron will either be equivocal or recommend exit (leaving his successor to negotiate what they can in the way of trade deals)? In that case, what are the chances of the electorate voting to stay in, when the right wing press that helped win the 2015 election for the Conservatives will be in full cry to leave? I would be foolish to say that exit was a probability, but I would be just as foolish to assume that the risk of leaving was small.
Voting for a political party that repeatedly puts itself before the national interest is not a good call in the best of times. When it could influence our position in Europe and even the Union itself, it becomes a huge mistake. Too many in the UK seem prepared to walk into that minefield, for the sake of avoiding what would be the mild inconvenience for them of a Labour led administration.
 I doubt very much that it will make any Labour government give additional preferential treatment to Scotland. The opposition will cry foul on this if that ever happened (and probably sometimes when it does not). As a result, Labour will go out of their way to avoid such an outcome. Would the SNP bring down a Labour government just because they failed to get some minor fiscal advantage? I think that is also highly unlikely. What the SNP will fear most is being seen as the party that brought down a Labour government and helped their opponents into power.