As election slogans go, it is hardly the sexiest or most memorable, but so proud are Labour of the phrase "Budget Responsibility Lock" that they have put it right at the front of their manifesto for 2015.

What the phrase means, according to Labour, is that every policy in its manifesto will be paid for without any additional borrowing, but more important in the coming three weeks is what could it mean for the respective fates of the different parties in the polling booths.

The answer depends on which election battleground we look at. In England, Labour still believesthat the election will be won on the middle ground and so is fighting to be seen as more fiscally responsible than the Conservatives. The promise of a Budget Responsibility Lock on the first page of the Labour manifesto is designed to underline that in red ink.

The phrase is also a conscious break with Labour's past, for better or worse. In launching the manifesto, Ed Miliband says that it is not a list of new spending commitments, as might have happened in the past, but a plan to secure the nation's finances with a guarantee that every single policy is fully funded and will not require more borrowing. He also contrasts it with what he characterises as the desperate spending promises by the Conservatives in recent days, and in doing so is seeking a reversal of the traditional positions of the parties. He wants the Tories to be seen as the party of irresponsible spending and Labour as the party of fiscal restraint.

The problem for Labour is that, even if such a message gets through in parts of England, the chances of it working in the very different Scottish battleground are much less certain. Mr Miliband says that his manifesto does something different in setting out a clear commitment that every policy in this manifesto is paid for without a single penny of extra borrowing and perhaps that attempt to out-sensible the Conservatives will work south of the border. But it highlights a significant difference with the SNP that has the potential to benefit the nationalists and damage Labour.

The difference is this: when asked, the leader of the SNP Nicola Sturgeon has said that, as part of a broad economic strategy, she would countenance some extra borrowing. Of course, she knows that the idea of more debt will go down badly with many Scottish voters and so has emphasised that any borrowing would be part of a continuing programme to reduce and eventually pay off the deficit.

However, the SNP is also making an explicit bid for Labour voters who are tired of austerity in the name of paying off the deficit and, if the polls are to be believed, the SNP message is working. In other words, Labour may believe that the message of "no more borrowing" could work for them in England, but it is a different message of "some more borrowing" that is working for the SNP in Scotland.

This fact that these differences exist in the political battlegrounds north and south of the border has created a real problem, and a dilemma, for Labour. The party knows that most of its votes will come from England, but it also knows that how it fares in Scotland could be critical to the final outcome. Direct your message to England, as Labour appear to be doing on the first page of their manifesto with their Budget Responsibility Lock, and it might win votes in the English middle ground. But what it is unlikely to do is win back many of those traditional Labour supporters who are still flirting with the SNP