John Rentoul: By September, the steel will show

David Miliband's support for Diane Abbott reveals an unsentimental willingness to undermine his brother's campaign

David Miliband has nominated Diane Abbott as a candidate for the leadership of the Labour Party. That is such a surprising sentence that it is still fun to type. It is as if the topsy-turviness of the Liberal-Conservative coalition has spread like a bad case of the giggles.

It is still strange to watch Nick Clegg sitting next to David Cameron in the House of Commons, doing what John Prescott did for Tony Blair, that is, nodding vigorously to give the Prime Minister cover on the left. It was downright peculiar to see Clegg in Berlin speaking in German on the British government's behalf, while William Hague, next to him, listened with headphones. It was a visual metaphor for the oddness of the coalition, as the pro-European Deputy Prime Minister was translated for his Eurosceptic cabinet colleague.

The coalescing of the Conservatives and Liberal Democrats seems to have liberated the Labour Party, allowing it to be more open about the fact that it, too, is a coalition. Ben Bradshaw, the former cabinet minister, was strikingly open on BBC 1's Question Time last week about the fact that he did not agree with Diane Abbott. The way he spoke of her, she might as well have been a member of a separate party. The Labour Party has always been like that, of course, but usually the niceties of party unity are observed in public. Now, transparency and openness are the motifs of the times. "I disagree with Diane" has become just as much a phrase of the moment as "I agree with Nick".

What is strangest of all about Abbott's candidacy for the Labour leadership is that ultra-Blairites such as me approve of it. When I was first a lobby correspondent in 1995, she and Dennis Skinner were the two members of the party's National Executive who could be relied upon to vote against the leadership. She had just commented on the vote on a new Clause IV of the party constitution by saying that most party members would "vote for the healing powers of cabbage" if Tony Blair asked them to, because they wanted to win the election. My first scoop for The Independent was the discovery that cabbage does indeed have healing powers.

So she has been ever since: a leftist would-be populist permanent critic of the leadership, but not very good at it. She was a rich source of news stories. Soon afterwards, she criticised "blonde, blue-eyed" Finnish nurses for taking jobs at her local hospital in Hackney. I reported that one of the Finnish nurses at the hospital was in fact black, and that the Miss Finland at the time was of Nigerian-Finnish descent. Abbott said she was "sorry and upset" that her remarks had been interpreted as racist.

Yet here we are, with David Miliband lending his support and that of several of his supporters to ensure that her name is on the ballot paper for the leadership. This is good for several reasons. One is that a wider range of views produces a better quality debate. It is healthier for a party that never felt that Gordon Brown had been tested in the fire of democracy. It is also healthy for the reason that Blair gave last week, when he was asked if it had not been a mistake to allow the Palestinian election of 2006 to go ahead, which led to Hamas controlling Gaza. "I don't think there's any point in not knowing that Hamas has support," he said. I suspect that the oppositionalist left has much less support in the Labour Party than most people think. Abbott is unlikely to get many votes from MPs or trade union members; and even among party members, who make up the third part of those to be balloted, she may not do that well. What she will do, of course, is take votes away from the most left-wing of the other candidates.

That is the other important reason for welcoming Abbott to the fray. It shows a flash of steel in David Miliband. The former foreign secretary has been unfairly cast as a serial bottler, for not challenging Brown for the leadership on a number of occasions when he would have failed to dislodge him because Labour MPs were too fearful. Now he has shown a streak of ruthlessness in claiming the mantle of pluralism while at the same time weakening his brother's chances.

Mind you, his brother is quite capable of undermining his own campaign with his ceaseless playing to what he thinks the party wants to hear – when there are many in the party who want to hear the summons of courageous leadership. Last week Ed called for the Government's fair pay review to be extended from the public to the private sector. If he thinks that party members or supporters really want a future Labour government to go back to the incomes policies of the 1970s, then he needs to be disappointed.

I hope that the Labour leadership debate so far is a reflex flushing out of the toxins that have accumulated in the body of a party that has not held a contested leadership election for 16 years. The posturing on Iraq and immigration is not going to impress the wider electorate. But the truth is that almost nothing is, so soon into the coalition's honeymoon. (Shirley Williams last week said that people were buttonholing her on the Tube to say that the Liberal Democrats had done the right thing.) That is why Labour is fortunate that it could not afford a quick leadership contest. The long campaign, with the winner to be announced at the start of the Labour conference in September, is good for the party. By the end of the process the candidates might have got down to the real issue, which is what Labour can say about the vast fiscal deficit with which it saddled the country.

But a long campaign also favours the candidate who has already shown staying power, and now steel. Abbott will be the "interesting" candidate for a few weeks, but I do not believe there is much appetite for her leap, rather than lurch, to the left. Ed Balls, who was surprisingly aggressive towards Ed Miliband, his fellow Brownite, in last week's debate, will do well. But there is only one candidate who looks like an alternative prime minister.

John Rentoul blogs at independent.co.uk/jrentoul