THE LABOUR PARTY is riven by the argument about when Tony Blair will — and should — step down. At the heart of this bitter row is a very simple question: would Labour attract more votes if Gordon Brown were leader instead of Mr Blair?

There is a clear, though complex, answer. A lot of polling has been done in recent months gauging how people would vote if Mr Brown were prime minister, evidence largely ignored in Labour’s factional argument because the truth it reveals is as unwelcome to Blairite diehards as to Brownites.

The polls show that Labour’s vote would increase if he were leader. But they show equally clearly that uncertainty about Mr Brown causes many voters to switch around with the effect that, though his presence increases the Labour vote, it pushes up the Conservative vote even more.

A recent Populus poll for The Times was the first to ask in a directly comparable way how people would vote in an election now, with Mr Blair, David Cameron and Sir Menzies Campbell leading the main parties, and then how they would vote at the next election if the leaders were Mr Brown, Mr Cameron and Sir Menzies.

The key conclusion is that the substitution of Mr Brown for Mr Blair causes remarkable volatility: more than one voter in five gave a different answer to the second question than to the first. In physics “Brownian motion” describes the random movement of small particles; the same, it seems, is true in politics. The prospect of the Chancellor being Labour leader causes a considerable churning of votes, from which six main clusters can be discerned:

1. The largest of these groups comprises people who would vote Labour in an election tomorrow, but say they don’t know how they’d vote if Mr Brown took over. About 4 per cent of voters fall into this category.

2. Nearly as many — about 3.5 per cent of voters — say they don’t know how they would vote in an election now, but that they would vote Tory if Mr Brown were Labour leader.

3. Roughly 3 per cent of voters say they don’t know how they would vote now but that they would vote for a Brown-led Labour Party.

4. About the same number say they would vote Liberal Democrat now, but that they would switch to Labour.

5. Fringe party voters scatter in different directions when presented with Mr Brown as leader; most become “don’t knows”, with slightly more switching to Labour than to the Tories or Liberal Democrats. About 4 per cent of voters are in this position.

6. The smallest group — 1.5 per cent of voters — would vote Labour tomorrow, but would switch to the Conservatives if Mr Brown were Labour leader.

This churning has the end result of pushing up Labour’s poll support by about 2 per cent. But the Tories end up about 5 per cent higher. The Lib Dems would be squeezed by about 3 per cent, as would “Others” by about 4 per cent.

This catalytic effect he causes has profound strategic implications for Mr Brown. Mr Blair had the freedom in opposition to reach out to centre-right voters without losing votes on the Left, because they were willing to put up with almost anything to get rid of the Tories. Mr Brown does not have remotely the same room for manoeuvre. If he tries to reassure voters on one side he will lose voters on the other; more or less whatever he does he runs the risk of losing at least as many votes as he gains. Many voters think that Mr Brown would be more left wing than Mr Blair, which attracts some and repels others.

Many others just aren’t sure about Mr Brown — who he really is, what he really believes. Despite nearly ten years as Chancellor, Mr Brown remains an enigma to most voters. This is partly a result of his knack for being absent when the Government has been in trouble — moments when voters tend to take more notice of politics. Most interviews he has given have come across as exercises in filibuster rather than an effort to project his personality and beliefs. Little clear impression of the man has slipped through over the years.

And for all that most voters think Mr Brown has been a pretty good Chancellor (though this reputation is steadily slipping), they do not think this necessarily means he would be a good prime minister; some even think the opposite, feeling that the characteristics they perceive in Mr Brown that help to make him a good finance minister — dour solidity, humourless asceticism, an English caricature of Scottish parsimony — are the wrong characteristics for a prime minister.

Having been Chancellor for nearly a decade, Mr Brown is tarnished in the eyes of many voters by the perceived failings of the Government, even while others look at him and fear the unknown. In a strange way he combines the disadvantages of familiarity and unfamiliarity. If Mr Brown is the new black, it is because for too many voters he represents a leap in the dark.

Andrew Cooper is a director of Populus, pollster for The Times

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