The agony of the tactical voter

I have a friend who would very much welcome your help. She lives in a marginal constituency and is agonising over how to cast her vote next month.

My friend describes herself as a left-of-centre progressive, and she has tended in the past to vote for the Lib Dems rather than Labour on the grounds that she trusts them more on civil liberties issues, to which she attaches great importance.  She was never a great fan of either Tony Blair or Gordon Brown, although she says she has been quite impressed by Ed Miliband.

Here's her dilemma. The constituency in which she will vote next month is currently held by a hard-working Lib Dem MP who has built up a good record as a vigorous campaigner on a wide variety of local issues: the library, the local police station, a proposed new waste disposal facility, the A&E department of the local hospital. My friend would very much like to vote for her again, but is terrified that if she does, the likelihood is that she'll have played a part in the re-election of a Cameron-led government. (Assuming, as she does, that if the arithmetic allows it, the Lib Dems would again do a deal, even if it isn't a formal coalition, with the Tories.)

This is her reasoning: to get rid of the Tories -- and she really, really wants to get rid of the Tories -- there will have to be lots more Labour MPs after 7 May. That means that voters like her, in constituencies where Labour are in a strong second place, surely should switch to Labour.

Trouble is, she says, it would feel like a betrayal. She hates much of what the coalition has done over the past five years, but she's pretty sure it would have been even worse without the Lib Dems holding the Tories back whenever they could. On the whole, she admires the way Nick Clegg has played the cards he was dealt by the electorate: she thinks he was right to go into coalition with David Cameron, but says enough is enough.

She also admires her local MP and would be genuinely sad to see her defeated. But she worries that unless she jumps ship and votes Labour, she'll hate herself for the next five years.

Here's what I've told her: if your priority is to ensure that David Cameron does not remain at Number 10, then yes, you should vote Labour. If you cannot forgive his government's reckless disregard for the most disadvantaged, both in Britain ("Austerity? The world didn't fall in, did it?") and abroad ("Migrants fleeing from the coast of north Africa? Let them drown, that'll teach them not to risk their lives"), then yes, you should vote Labour.

If you want to avoid the nonsense of a referendum on Britain's membership of the European Union (a referendum that would owe everything to internal Conservative party divisions and nothing whatsoever to the national interest), then yes, you'd better vote Labour.

I suspect what my friend would most like is to be able to vote Lib Dem again and see a post-election coalition made up of Labour and Lib Dem MPs, maybe with external SNP support. But she knows that for that to happen, there will have to be more Labour MPs, and there won't be unless enough people vote Labour.

I've tried to be fair, so I've also given her some reasons why it might make sense to stick with the Lib Dems. You could argue, I've told her, that in an election as uncertain as this one, the best thing to do is to cast your vote where your heart tells you to cast it, for the party you trust that has the ideas you agree with.

If you like what the Lib Dems say about introducing a new media freedom law, vote Lib Dem. If you think hard-working constituency MPs are important, then again, stick with your Lib Dem candidate. If you admire what they've done on civil liberties (surveillance and ID cards, for example) or on taking the low-paid out of income tax, then yes, vote for them again.

But I'm a hard-hearted pragmatist, and I have also reminded my unhappy friend of a simple truth: the party -- or parties -- that form the next government will be those that have gained the most support on 7 May. And unless more people vote Labour -- and fewer people vote Conservative or Lib Dem -- Mr Cameron and Mr Osborne will be back in office for another five years.

So that's what I've told her. What would your advice be?

(In the interests of full disclosure, I should make clear that my friend may not actually exist. She may be a mere journalistic device to illustrate a point -- but she'd still very much welcome your thoughts.)