Get real. Keep your head screwed on. What result do you want? I will assume, dear Guardian reader, that like me you have two prime purposes. One is to prevent Cameron walking into Downing Street on 7 May. Equal first is to secure electoral reform so that we are never again presented with such a disgraceful voting choice. If that's not your view, you can save time, stop reading here and push off to some Murdoch organ that will amply satisfy your needs.

If you are still reading, then the choices on offer are somewhere between negligible and zero for almost everyone. How to vote all depends on where you vote to keep Conservatives out and bring electoral reform in. Start here: Ben Page, the head of Ipsos Mori says he will "run naked through the streets" if Nick Clegg wins this election. That might be entertaining, but it's not going to happen – or not on anything like present polling. Stop and think long and hard about this. Ipsos Mori does not stake its good reputation lightly on such predictions. Do not expect Mr Page to be doing his Lady Godiva impression any time soon.

By all means tell pollsters that you will vote Liberal Democrat, but unless you succeed in getting the opinion polls to show the Lib Dems at 40% or more – or some other extreme configuration – Nick Clegg has no chance of becoming the next prime minister. None, zilch, zero. I can't find a pollster who thinks that he will go over that critical threshold.

Hundreds of posters on Guardian threads yesterday wanted a Liberal Democrat win. No surprise, since there is no doubt that Clegg was the best in both debates. He talks human not slogans, he is sincere, and he stands by unpopular positions without trimming. Those who squirmed at Gordon Brown asserting that Trident will protect us against Iran and North Korea – or his failure to answer on what to do about long-term illegal migrants, their children and, soon, grandchildren: stateless, exploited and paying no taxes – will think Clegg a brave and decent man. So he is on human rights, and how to "punch above our weight" in Europe. He might make a good prime minister. But it won't happen this election because the abominable voting system makes it impossible. Win electoral reform and he might win next time, if there is a rerun after we have got voting reform.

But until that day, never take your eye off the ball, whatever it takes. If many go with heart not head against such ferocious odds, and Cameron gets the crown, electoral reform will be a dead duck, and it's back to square one. Siren voices saying that "they're all the same" forget or weren't born in the 1980s. I have never much minded what the best anti-Tory party is called, I just want the left of centre to win. I will always back whichever group combines being furthest left with winnability: that's always a trade-off. Never let Conservatives win by siding with the unwinnable because you like the cut of their jib a bit better. Don't let the best be the enemy of the tolerable. Don't be tribal, stay focused on what the polls say and don't indulge in: "If only everyone voted like me, we'd win."

It's not a game. The people who always, without fail, get hit hardest by Conservative governments are the powerless, the weakest, the voiceless – and they may not even vote. It is an absolute certainty that inequality will always worsen under the Tories, always did, always will.

This time it will happen faster: Labour tried hard – not hard enough – and even then slipped back slightly. No doubt Cameron and Osborne would prefer it didn't happen. No doubt they would be embarrassed by the inevitable sudden rise in poverty – but they have no idea how much they need to keep redistributing through the next harsh years to match Labour's record.

They have no plan to keep paying into anti-poverty programmes, especially Sure Start. They wouldn't be Conservatives if that was their priority, and their Big Society can never fill the void. Those who enjoy some of the nihilistic, mocking coverage of this election need to remember that. Politics matters, maybe not much to my life – but to the life chances of many millions.

So what is the anti-Tory voter to do now? What do you do when the old two-party electoral system has finally collapsed into a genuine three-party contest? Look at the result according to yesterday's BBC poll of polls: Clegg gets 30% and a puny 102 seats, Cameron gets 33% and only 258 seats, while Brown comes third with 27% and emerges as the victor with 261 seats. Every time you see a poll, go to the BBC's brilliant election seat calculator for a nasty shock. Work out any variety of options. Labour may yet do far worse – but if so, Cameron wins, not Clegg.

Compass, one of the most imaginative of forward-thinking Labour groups along with the Fabians and Progress, has been pushing for proportional representation long before the cack-handed cabinet came round – too little, too late – to their pathetic alternative vote pledge. Compass is balloting members today on whether to advocate tactical voting. Savvy readers will have been voting tactically for years – but for a group more or less within a political party to advocate that Labour members should switch to the best anti-Tory chance in their constituency is a brave act. Many have been throw out for it. Ask the wise and defiant Labour members in Lewes, East Sussex, who were ejected for supporting their good Lib Dem MP, Norman Baker, in a seat where Labour had no chance.

Braver still, the Labour MP Jon Cruddas – a Compass leading light – has sailed as close to the wind as possible today in almost endorsing it: "I'm urging everyone to vote Labour, but it's clear that politics is changing before our eyes and we cannot be in denial about that." Indeed not. Every anti-Tory voter – Labour, Lib Dem, or of no fixed abode – needs to get real about where we are, within tantalising reach of radical reform.

Here we stand on the shifting sands of a fundamental and lasting change. Believe me, I would rather write here about the political choice of my dreams, a high-flown vision of a golden dawn. Voters plainly yearn for something better. But I'm sorry, unless some second earthquake lifts the Lib Dems over that impossible threshold, all progressives are stuck with the same old choice. It's low tactics, not high romance. Vote what best keeps the Tory out where you are. Buck that arithmetic at your peril.