Allow me to introduce you to the Four-Square Rule. The Conservatives need to be four points ahead of Labour in share of the vote if they are to remain the largest party in the House of Commons. A four-point Tory lead is therefore where the parties are "all square". It is the break-even point between the Tories having the most seats, in which case David Cameron stays on as Prime Minister, and Labour overtaking them, in which case Ed Miliband moves in.

As the year ends, the Tories are on average five points behind in the opinion polls. That should not be too discouraging for Cameron. To go from five points behind to four points ahead seems a less than Herculean task, especially if voters start to feel better off next year, which seems likely.

Except that there is a bluebottle in the ointment – Nigel Farage. Number 10 was spooked last week by a full-page advertisement in The Daily Telegraph taken out by Alan Bown, a UK Independence Party donor, who has paid for polls in eight target seats. The polls were scary for numerate Conservatives, suggesting that the Labour vote was up by more in Conservative-Labour marginals than in national polls; that the Tory vote was down further than would be expected; and that Ukip was doing better than expected. In some places, this would hand Tory seats to Labour, with Ukip pushing the Tory candidate into third place.

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Certainly, the Conservatives should take Ukip seriously. Lots of people want to vote Ukip, and for reasons which are not as unrespectable as is often pretended. Many of the party's policies are nonsense, as anyone who has tried to reconcile its spending promises with its plans to cut taxes and borrowing can attest. But the impulse behind them is widely shared. Ukip is more than a conservative party, it is a party of reaction. It wants to go back to before 1971, when the UK Parliament could decide who was allowed to live in this country, when there were grammar schools (and secondary moderns) everywhere and marriage was heterosexual.

Farage is a brilliant leader of this popular emotion. Like Alex Salmond and George Galloway he is a separatist, adept at promising to separate people from reality. Like them, he is poorly understood by Westminster journalists, which is why we anticipated the SNP victory in Scotland in 2011 so poorly and Galloway's win in the Bradford West by-election not at all. The European Parliament elections next year will give Ukip a good platform. "Half of the Tory tribe goes off and supports Ukip," says a senior, gloomy Conservative. They treat European elections "like a weekend retreat".

It will test the Prime Minister's nerve, but there is a limit to populism. I hope that the Scottish people will show in September that an anti-London vote is one thing, independence quite another. Galloway cannot get himself elected in the same seat twice. And a year after Ukip won 17 per cent of the vote in the European Parliament elections in 2009, it secured 3 per cent in the general election. Ukip has money and members but not much time to build an organisation capable of electing MPs.

Then Farage comes up against another limit. The more successful Ukip is, the harder it is for him to maintain the anti-politics pose. His dinner with Rupert Murdoch in March made him look like just another politician; and it is notable that he rarely mentions same-sex marriage, although it really animates his new recruits.

His reluctance is part of his desire to keep his party away from some of the fruitier cakes of the better yesterday brigade, such as Godfrey Bloom, the former Ukip MEP who yesterday accused Farage of having done a deal with the Tories to stand aside in some seats in return for a peerage. (As if: the party would lynch him.)

However, Ukip is still a problem for the Conservatives. Bown's polls confirm that Ukip has taken twice as many voters from the Tories as from Labour. But Labour is unlikely to win Thanet South as the result of a Ukip surge. If Farage stood, he could affect the result in one seat, but polls in individual constituencies have a poor record of predicting general elections, especially this far in advance.

National opinion polls are a better guide. In them, Ukip's support currently averages 12 per cent – a long way from "breakthough". And if Ukip does not break through, the only thing that matters is whether it takes more votes from the Tories than from Labour. The crucial point is that the damage Ukip has done to the Tories has already been done, and the Tories are only five points behind Labour. The question is whether Ukip will win over even more Tory voters rather than Labour ones between now and the election. I don't think so.

Certainly, by the end of last week, Cameron's advisers had calmed down about those Ukip polls, saying that they "hadn't changed the price of fish very much". They have to say that, of course, but I think they are broadly right. The success of Ukip makes it a harder for Cameron to get four points ahead of Labour, as required by the Four-Square Rule, but that is still an achievable target.

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