Campaigning requires focus. Having defined your objective, you design a strategy to achieve it – a strategy that you must stick to as tightly as possible. The language of that strategy, its techniques and tactics require repetition and discipline to communicate successfully to any audience – all the more so when your audience is an entire nation, and the nation in question is as sceptical of politicians as Britain currently is.

Factors which disrupt and distract from your strategy, therefore, are serious threats. We saw in the AV referendum that the Yes camp never seriously got round to making their own case for any sustained period of time, because their opponents successfully drove them onto discussing the weaknesses of the system. That was good work from No2AV, but they were aided in doing it by the Yes campaign’s willingness to be diverted from their strategy.

Labour currently face just such a disruptive threat – not so much from their main opponents (though we will see what possible grenades George Osborne may lob into their trench on Budget day) but from another source: the SNP.

The shortest route to a parliamentary majority for any party begins with holding all of their existing seats, before seizing new seats from others (ideally from the other possible party of government, where a gain of one means an increase of two in your parliamentary majority). In Labour’s case, only a few months ago they would have confidently predicted that they would retain their raft of MPs from Scotland. Now, as Lord Ashcroft’s latest snapshot suggested, they are facing the possibility that the SNP could seize a sizeable proportion of those seats.

That makes the arithmetic much more challenging for Labour, but the real damage may be in the impact on their strategy and their operation.

Strategically, things aren’t now as simple for Miliband as keeping quite about the deficit and suggesting the evil Tories will sell the NHS down the pawn shop for £20 and an Eton Old Boys’ tie. Suddenly Labour face the complexity of dealing with a centre right opponent and a nationalist threat in Scotland which attacks them from the left.

It may sound like a mirror image of the Tory challenge in seeking to fight Labour and UKIP – in reality it’s worse. We all know the serious potential for disruption by UKIP, but the darkest Tory nightmares involve a handful of seats going purple and Farage’s “army” helping Europhile Labour or Lib Dems into Parliament by dividing the vote on the right. The reality in Scotland is that it seems Labour may lose most of a historic heartland directly to an insurgent party – the equivalent for the Conservatives would be polls that show UKIP leads in most seats in the South East of England.

Imagine the chaos involved if that situation came to pass . Money would be diverted from winning targets or holding marginals to defending seats that had been though safe. MPs and Ministers who had planned to tour the 40/40 rallying the troops would stay on their home turf, fighting to survive. In TV appearances, rather than delivering messages designed to appeal to Lib Dem voters in the South West, spokesman would be speaking to those who were wavering on the right. The leader would be justifiably concerned about standing near high windows, lest his own MPs decide to push him through one.

All that is going on inside Labour at the moment. The prospect of troops and cash being shipped in from Scotland to fight English marginals is a distant memory. The many Scottish MPs who hold senior roles in the Labour machine are understandably distracted from the main task at hand by the prospect of losing not just the election but their own jobs. The strategy with which they began the campaign is now under question, and various people are testing out rogue messaging that they feel is more appropriate.

I wrote recently about the (admittedly uninspiring) reality of this campaign – that there is a real prospect that one side may lose, rather than the other necessarily win. Sticking to your strategy and maintaining message discipline doesn’t guarantee you victory, but abandoning both certainly makes defeat more likely.