Here’s an interesting concept to get your head around. What are the chances of diehard Labour supporters voting Conservative for tactical reasons? Higher than we think.

The emergence of a political force from the right such as UKIP, with its policies so much like Kryptonite for many Labour supporters, may give rise to some interesting voting behaviour among them.

We saw a bit of it in the Rochester and Strood by-election last month. The Tories performed a lot better than expected, losing by a much smaller margin. The explanation observers came up with at the time was that many Labour supporters, understanding that it was a two-horse race between UKIP and the Conservatives, had not wanted UKIP to win enough that they held their nose and voted Conservative.

That doesn’t mean, of course, that this will happen in the General Election. By-election voting behaviour has always been different, with voters making localised decisions, and tending to understand that their decision isn’t going to affect the government of the country. Somehow thinking that Labour supporters would actually help the Conservatives win a General election by helping them win seats in Conservative/UKIP marginals.

Yet there are two reasons why I think it could be possible. The first is ideological and the other more likely reason is tactical.

1) Ideological – It is difficult to overstate the disgust of the liberal middle class intelligentsia at the rise of UKIP. It is visceral and it is bordering on the legal (in that given a chance many Labour supporters would happily legislate for UKIP not to be given a political platform). That being the case, some people who vote Conservative should Labour not be in contention even if it means they get more seats than Labour in this election just to try and kill off UKIP’s chances of getting a foothold in Parliament. I can’t say how many would do that, but I do think it is more than many realise.

2) Tactical – Labour are genuinely worried about UKIP defeating them in some of their ‘heartland’ seats in the north. They are up against a political party who can say anything they want as they are extremely unlikely to be in power. UKIP can tell people that the answer to their life’s struggles involve shutting the door to immigration and leaving the EU and enough people may believe them to vote for them that Labour could lose quite a few seats to them.

However, UKIP only have so many resources for fighting year’s election. They can ‘park their tanks on Labour’s lawn’ in addition to fighting Conservatives in key marginals only if they don’t have to spend too much time and money on those battles with the Conservatives. Given it is a harder sell to get people voting UKIP in Labour heartlands than it is in Conservative ones, they need to carefully husband their resources to be able to fight on those two fronts.

Unless the Labour supporters in those Conservative/UKIP marginals make it clear they will be voting Conservative.

If that happens, UKIP will need to commit more resources there and less to Labour heartlands and Labour actually win more seats than they would have done.

Either way, if you are trying to predict what might happen in key marginals in this country, don’t just look at the two parties involved, look also at the other parties, particularly if UKIP has a chance.

Conservative victory really could hinge on whether Labour voters hate UKIP more than they love Labour.