If Clacton were marginal between Labour and UKIP, a rational option for me would be to vote for Douglas Carswell, Mr UKIP Moderniser himself, in order to keep Labour out.  I am reflexively hostile to Ed Miliband’s party, and like, admire and am amused by Carswell in more or less equal measure.

To take a more likely illustration, were UKIP a strong second in Birkenhead, a choice to suit my preferences would be to vote for Frank Field.  I admire Field and dislike UKIP.

Or to take a real-life example, were I a voter in Inverness, Nairn, Badenoch and Strathspey, a practicable course to take would be to vote for Danny Alexander.  The SNP are challenging in the seat. Labour was second there last time. I think Alexander has worked well with us: party members seem to agree.

If I, a party animal, can see the point, then so will very many other voters who are less fixed in their loyalties.  They will vote tactically in May – if they turn their minds to the possibility, decide that it’s for them in their particular seat, and feel that they have enough knowledge to make a informed choice.

That this will happen is nothing new.  The Liberal Democrats would not have broken through in 1997 without anti-Conservative switching on a big scale by Labour supporters and others.  But the sheer scale and depth of Lord Ashcroft’s polling may make it even more prevalent in a few weeks’ time.

Good luck to anyone who chooses to take that option.  This is a Parliamentary democracy; that is their democratic right.  But although I have given reasons for voting tactically in some specific seats, there are two others that countermand them.  One comes from the head, the other from the heart.

The first is that if one has a strong party preference, one should want to build up one’s party’s long-term position in apparently hopeless seats.  This would counsel against voting for Messrs Carswell, Field and Alexander.

I cannot put the second better than Alistair Burt did in an article about football on this site.  He discussed which team one should support if one’s own can’t win, turned to politics, and wrote as follows:

“In a Haringey Council election many years ago, my wife and I were to vote in Tottenham. Our candidate was nowhere, but the Liberal had a chance of beating Labour. We resolved to vote that way. As we emerged from the polling station we looked at each other, and burst out laughing. We had both voted Tory. We just couldn’t do it.”

P.S: The Conservative candidate in Clacton is the excellent Giles Watling.