I live in Holborn and St Pancras, a Labour seat — how should I vote? According to the Lib Dem candidate Jo Shaw, the incumbent MP Frank Dobson has a majority of 4,787, the Lib Dems being second with the Conservatives trailing far behind. Would it therefore be sensible to vote tactically for the Lib Dems?

I tend to think not. Holborn and St Pancras is number 69 on the Lib Dem list of target seats. If the Lib Dems won the preceding 68, they would have 130 seats in the new parliament. What would that mean? Let’s start from the current UK Polling Report poll average: Con 38, Lab 31, LD 20, Others 11. As all of the LD target seats are presently held by either Con or Lab, we can leave Others at 11% of the vote. I also assume that the Con/Lab ratio remains constant at around 1.23. According to the UK Polling Report uniform national swing calculator (and keeping to whole numbers in the national vote percentages), in order to preserve this ratio and give the Lib Dems close to 130 seats the General Election result would have to be roughly Con 31, Lab 26, LD 32. The seat numbers would be Con 232, Lab 259, LD 128. (Note that I have no idea whether the calculator’s results are valid for this kind of move.)

Of course, there is not a uniform national swing — some constituencies swing more than others. The numbers above can therefore only give a flavour of the kind of world we would be living in if the Lib Dems took this constituency. But they do illustrate the fact that if the Lib Dems did actually win this seat on account of a sudden surge in their support, the Conservatives would likely have fewer seats than Labour in a hung parliament, but a greater share of the national vote. In that case, the larger the Conservative share of the national vote, the stronger the case the party would have for being the Lib Dems’ chosen coalition partner in spite of having fewer seats than Labour. So it makes sense for me to vote Conservative.

What about a situation where Labour commits some terrible error and is annihilated, perhaps even coming third in terms of the national vote, the Conservatives are the main beneficiaries, and the Lib Dems do well enough to take this constituency. Then, if I voted Lib Dem, my vote might count in kicking out the Labour incumbent here. But there is no reason to think another Lib Dem MP would help the Conservatives in that situation. The Lib Dems would be newly assertive while Labour would be shattered and self-absorbed, so it might be better to have one fewer Lib Dem MP to oppose David Cameron rather than one more.

Conclusion: Remain Tory!