Andrew Grice: Milibands are brothers on top

They enjoyed a Christmas Day lunch together at Chevening, the Foreign Secretary's country residence in Kent. They send each other advance copies of their speeches for comments and text each other frantically when their government work overlaps. The Miliband brothers are close.

But after David Miliband's autumnus horribilis, some Labour MPs are asking whether his younger brother, Ed, will overtake him in the future Labour leadership stakes, and even whether the Cabinet's band of brothers might become rivals for their party's crown.

Their close relationship was tested for the first time when David, the Foreign Secretary, launched a thinly-veiled attack on Gordon Brown's hapless performance as Prime Minister in July and made clear he would be a candidate to succeed him if he were forced out. Ed was torn: he felt he owed everything to Mr Brown, for whom he worked as an adviser before becoming an MP, and had to remain loyal to him. If the Prime Minister had been toppled – which looked very likely for months – he would have supported his brother in the subsequent Labour leadership contest. But he could never have been part of an anti-Brown coup.

The storm threatening to blow away Mr Brown passed, and the tension level among the Milibands subsided too. When Ed was promoted to Energy and Climate Change Secretary in October, David was a useful source of advice on both policies and personalities. He used his knowledge as a former environment secretary to help his brother set up his new Whitehall department, never an easy ride. In a neat piece of symmetry, Ed oversaw the passage into law of the landmark Climate Change Bill to cut Britain's carbon emissions – which had been introduced by his brother.

Officials noted their closeness during their "frantic texting" when they represented Britain at international talks on climate change last month. Ed was at the UN-led negotiations in Poznan, Poland, which overlapped with the European Union summit in Brussels attended by his brother. Their constant messages ensured other countries did not pull the wool over the eyes of British negotiators.

Despite that, the potential threat to the partnership between the two sons of the late Marxist historian Ralph Miliband has not gone away.

Labour MPs have been, in their own words, "selling shares" in David Miliband after his second false start in a leadership race. In 2007, he came under pressure from Blairites to stand against Mr Brown when Tony Blair departed, but decided against it. Last year, he put himself in the frame but was left stranded at the starting gate when Mr Brown recovered. David endured an embarrassing Labour conference in September. Everyone knew the Prime Minister's attack on David Cameron as a "novice" was also a dagger aimed at him. He was photographed holding a banana, an image that may stick to him like William Hague's baseball cap.

Some Labour MPs are "buying shares" in James Purnell, the Blairite Work and Pensions Secretary, who would have backed David Miliband last year but might emerge as a candidate in his own right next time. Others, however, are talking up the chances of Ed Miliband, and he has just entered the bookmakers' list to be prime minister at the end of 2009. "Ed is doing well in the climate change post and there is a real chance he will overtake David now," said one senior Labour backbencher. "There is no doubting his ability or ambition. There is a view around that David has had his chance and missed it. Gordon is there at least until the election and that gives Ed time to come through. He is the one to watch."

It is true that politics is a game of Snakes and Ladders. At the end of a dramatic year in politics, Ed is up and David is down. But allies of the Foreign Secretary insist he will bounce back and be a contender in any future leadership contest. "There was no leadership campaign," insisted one ally. "No one's career was ever ended by a banana." With the Prime Minister safe until the general election, the Foreign Secretary is keen to enter the debate about Labour's future policies – to focus on ideas, not personalities.

As with Mr Blair and Mr Brown in 1994, mutual friends cannot contemplate the Milibands running against each other. Some predict that Ed, who was 39 on Christmas Eve and is four years younger, would defer to his brother one last time. Another reason why he might hold back is that Mr Brown might favour another Ed – the Schools Secretary, Balls – as his successor.

But it would all depend on the timing and circumstances, Labour MPs believe. If there were a hung parliament, for example, Mr Brown could lead the party for at least another couple of years, which might boost Ed's prospects.

Although it is difficult to imagine the two Milibands standing against each other, they are not the same person and do hold different views. David regards himself as slightly to the left of Mr Blair, even though he would carry the unwanted label of the "Blairite candidate" in any leadership election. Ed, in turn, is seen as slightly to the left again of David, winning plaudits from left-of-centre groups like Compass for the radical rallying cries he delivers at their conferences.

Despite their bond, and the fact that they live round the corner from each other in Primrose Hill, north-west London, the brothers do not see as much of each other as they would wish. Their different government posts often keep them apart.

Still, they have had some precious time in which to catch up over the Christmas break. It is unlikely that the games played at Chevening included Snakes and Ladders. That might have been a little too close to home after their contrasting fortunes in 2008.

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