John Rentoul: Brown is safe, but not home and dry

The Prime Minister had a good week, but the murmuring of leadership challengers is always audible

Gordon Brown likes his dividing lines, and Labour's new bash-the-rich theme is in big black marker pen. This week the pre-Budget report will tax the rich; last week the bonuses row was a chance to bash the bankers.

This is a dividing line all right. First Peter Mandelson was on the radio to say that City bonuses were all very complicated and that RBS should not be "singled out" just because we, the taxpayers, own it. Then Harriet Harman was up in the House of Commons, saying: "I'm happy to condemn those who simply have no recognition of the fact that people expect the banks to play a part in the economy." She didn't include the Business Secretary by name, but her words – "recklessness and irresponsibility" – could not have sounded more different.

What is going on? Well, most of it is no doubt chaos and confusion; in other words, politics as usual. Mandelson and Harman have long been at odds, in both ideology and style. It was Mandelson's head that hit the cabinet table with a theatrical thud at the start of this year when Harman said she knew that the point she was making about a third runway at Heathrow was "irrational". But the background noise in politics is of surreptitious competition for the top job.

No, I do not mean Lord Mandelson. The Bill that might have allowed peers to resign from the House of Lords, thus enabling him to re-enter the Commons and stand for the Labour leadership, was not included in the Queen's Speech. Even so, his comments in the same radio interview last week have been misinterpreted. Many people expressed surprise that he admitted that he "would of course have liked" the European High Representative job, which went to his successor as Britain's commissioner, Cathy Ashton. That was taken as a death rattle of the Brown administration: even the Prime Minister's right-hand man wanted to leave the country. I think the real significance was the opposite: that Mandelson is committed to the Labour project, however attractive the continental option. Politicians, eh? Meaning what they say – whatever next?

I meant Harman. She is the one more likely to have an eye on Brown's job. She has ruled herself out most emphatically, but it is possible that, unlike Mandelson, she does not quite mean what she says. One Labour person who has known her since she was legal officer of the National Council of Civil Liberties tells me: "Harriet has always wanted to be leader of the Labour Party." Some of the Blairites in the Parliamentary Labour Party are worried about her.

It all seems too late for Labour to change leader, and we are certainly approaching the endgame. Brown has had a good week. Not only did he "win" at Prime Minister's Questions against David Cameron but, more importantly, he and Alistair Darling have resolved their tensions over the pre-Budget report. This is a pity, from the point of view of the Labour Party and the long-term interests of the country: a big bust-up on economic policy between Nos 10 and 11 Downing Street is the traditional trigger for a coup against a sitting prime minister.

There has also been much comment about a narrowing in the opinion polls. This reminds me of the many similar headlines in the Daily Mail during the early Tony Blair years, along the lines of "Labour slumps to 12-point lead". Now Cameron's average lead in the polls has slumped from 14 points to about 12. Some polls have put the Tory advantage at a mere 10 points, and there was one outlying Ipsos MORI poll last month that put the gap at six. Now, a 10-point Tory lead produces, according to the conventional calculations, something very close to a hung parliament, which has lifted Labour morale.

There are, though, reasons to be not so cheerful. A simple point about the coming election is that it is going to be fought in Labour-held seats. Despite the higher than usual number of MPs that are standing down, it is also going to be fought largely by incumbent Labour MPs, all facing candidates who will ruthlessly exploit the expenses issue. So, while the opinion polls are moderately encouraging for Labour, national opinion polls may not translate into seats in such a favourable way, and the party is in danger of taking the wrong lesson from the figures. It should not fall into the trap of thinking that, under Brown's leadership, it could deny Cameron a majority. It should draw the conclusion that it is more likely to be able to do so under a different leader.

The one person who does not think it is too late to change leader is Brown himself. Like all prime ministers, he fears that he could be out of the job by the end of the week. Every week. That may help to explain why his people briefed against the Foreign Secretary at the Commonwealth meeting in Trinidad last weekend. Journalists were told that David Miliband had tried and failed to persuade the Sri Lankans to withdraw their bid to host the next meeting; Brown then succeeded.

If he is worried about Miliband, then so are Miliband's supporters. They are worried that Miliband "doesn't want to do a Heseltine", as he said when he pulled back from a challenge to Brown last year. That is to learn the wrong lesson from history, namely the old saw that he who wields the dagger never wears the crown. But the reason Michael Heseltine was unpopular with the Tory membership was that they idolised Margaret Thatcher. Labour members do not feel the same way about Brown.

That is why the Blairites are alarmed by Harman. Suppose, they say, that the polls fail to move further in Labour's direction; that Harman tells Brown that she thinks he should go; that a grateful party begs her to go back on her repeatedly expressed refusal to consider the top job. I hope that this is simply a nightmare conjured by Blairite ultras to scare David Miliband into doing what is needed.

Brown had a good week, but that does not mean he is safe yet.

John Rentoul blogs at www.independent.co.uk/eagleeye