Leading article: Volatile voters, resurgent Tories and the decision to call an election

If last week was a triumph for Gordon Brown, this week indisputably belonged to David Cameron. The Conservative Party put aside recent divisions and presented a united front at its annual conference in Blackpool. The tax plans announced by shadow Chancellor George Osborne were favourably received. And Mr Cameron delivered an impressive speech, demonstrating that he does not wilt under fire.

At the same time, Mr Brown undermined his carefully crafted new image. His surprise visit to Iraq to announce troop withdrawals in the middle of the Tory conference seemed a rather unstatesmanlike stunt. The flurry of government activity before the recall of Parliament may also have backfired on Mr Brown. Lord Darzi's health review was brought forward. The City of London was bounced into making a decision on the London Crossrail project. And yesterday Mr Brown moved forward the Government's Comprehensive Spending Review.

All this has served to confirm that talk of an election is not mere mischief from Labour. It is clear the Prime Minister has been considering calling a snap poll. But has Mr Brown painted himself into a corner? If he decides against calling an election now, he risks looking as if he has marched his troops to the top of the hill and promptly marched them down again.

So should Mr Brown call an election? It is a very fine judgement call. The Conservatives and Liberal Democrats have been demanding a poll. And an election would give Mr Brown a chance to secure his own "mandate" to govern. It is also highly unlikely that the Tories would win a majority in parliament, despite their fight-back. The Liberal Democrats are struggling too. Sir Menzies Campbell failed to get much of a boost from his own conference a fortnight ago and is being squeezed by his two pugnacious rivals. It looks as if Britain is returning to the era of two-party politics – although, perversely, if there is an election a hung parliament is not out of the question.

Yet the biggest problem for Mr Brown is one of predictability. It is not worth the Prime Minister going to the country early unless he can be confident of increasing Labour's majority. Anything less than the present 66-seat majority in the House of Commons would look like a moral defeat for Mr Brown; he would be sacrificing seats without cause. But predictability is in short supply in the present political environment. One reason is that no one knows how constituency boundary changes, which would come into effect if a snap poll is called, will affect the dynamics of a general election. Another problem is turnout. Polls in autumn are rare. Prime Ministers almost always call them in spring when the weather is likely to be reasonably good. Cold weather and dark evenings in early November would impact on campaigning and could mean a lower turnout, something that historically has tended to depress the Labour vote in particular.

And then there is the volatility of the swing voters. Last week the Labour poll lead looked unassailable. But in just seven days it has been cut back dramatically and Mr Cameron has recaptured the cherished prize of political momentum. Mr Brown is confident next week's spending review will restore that lead, and that he can demolish Tory tax plans during the heat of an election campaign, but Mr Cameron's telegenic appeal could easily switch momentum back again in his favour.

No one knows if a poll will be called. Mr Brown himself is probably still undecided until he has weighed all the evidence this weekend and consulted his closest advisers. But after this week's Tory fight-back one thing is for sure: Mr Brown would be quite foolish to underestimate Mr Cameron.

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