John Rentoul: Tony thinks big. Gordon thinks of Mandy

Blair swoops in on his high-carbon flight to battle climate change, leaving low-carbon Brown green with envy in his wake

As the PM travelled to Brussels by low- carbon train, Blair travelled by plane to Tokyo to remind us how saving the planet should really be done

In Brussels, Brown carried on a 14-year-old feud with Peter Mandelson, the European Trade Commissioner. In Tokyo, Blair lifted his eyes to the future, offering to break the global "deadlock" in climate talks by high-altitude diplomacy between Japan, China, India, Europe and the United States.

Once again, Brown must reflect on the unfairness of politics, and his resentment against Blair once again finds its target in the proxy form of Mandelson. He went to Brussels to try to negotiate the practical measures to achieve the changes that Blair says are urgently needed. Yet his press coverage is of an ancient vendetta and, if he is reported on green issues at all, it is for his views on the taxation of fridges.

Meanwhile, Blair is hailed for his stirring summons to a global "climate revolution".

The story of last week's scorpion-dance between Brown and Mandelson is convoluted. A year ago, Mandelson ruled out serving another term in Brussels, because he thought Brown would never appoint him. But he could not resist adding: "He can't actually fire me so, like it or not, I'm afraid he will have to accept me as a Commissioner until November 2009."

Last week Mandelson realised that, in the new mood of inclusiveness sweeping Brown Central as a way of fighting back from the disasters of the autumn, he might have a chance of serving a second term after all. An account appeared in the press on Monday of a conversation in which Brown asked Mandelson if he had thought about a second term. A source was quoted as saying that Mandelson "would certainly think about it" if it were offered, and that he had not really meant to rule himself out.

When the Prime Minister arrived in Brussels on Thursday, however, he told journalists: "Mr Mandelson has said he wouldn't be serving a second term." Ouch.

That might not have seemed so petty had Brown arrived in Brussels with some higher purpose to pursue and some great advance to proclaim. He tried, but his plan for Europe-wide tax cuts on green goods, outlined to journalists on the train, was quickly brushed aside by other European leaders when he got there.

Meanwhile, Blair sailed above him through the troposphere to a grander world stage, the "G20" group of 20 industrialised nations in Tokyo, where he set out a green mission that was striking in its simplicity. "If the average person in the US is say, to emit per capita, one tenth of what they do today and those in the UK or Japan one fifth, we're not talking of adjustment, we're talking about a revolution."

This is Blair in what I call his 1968 mode, talking the heady talk of transformation, of (in his speech yesterday) "the critical moment of decision".

Who could argue that, while Brown was wading through the EU rules on VAT on cleaning products and insulation materials, the ultimate battle to avert climate catastrophe depends on a global deal, one that includes America and China? And who could argue that one of the people best placed to try to broker that deal is Blair himself?

That is Brown's eternal dilemma. He thought he would be free of Blair's shadow once he got to No 10. It was partly the dawning realisation that this might not be the case that made him so desperate to secure his own election mandate in the autumn – so much so that he dithered his reputation for prudence away.

Yet Brown knows that he cannot escape Blairism. How can he? He helped to create it, and the alternative is a leftward lurch to electoral disaster. Thus it is that Blairites such as John Hutton, the Secretary of State for Business, can push their luck where Mandelson tried and failed. Last week Hutton gave a speech in which he said: "It would be a good thing for our country if there were more millionaires in Britain not fewer."

Tonally different from the hairshirt ethos of Brownian-Darlingism, and electorally more appealing. That is why the Blairites in the Cabinet, Hutton, James Purnell and David Miliband, are thriving.

It is not just Brown, or his Cabinet, but the whole Labour Party that knows, at some deeply hidden level, that there is no going back. It may be reading too much into one parliamentary selection, but last week Christine Shawcroft failed to become the Labour candidate to succeed the hard-left Alan Simpson in Nottingham South. She was beaten by 101 votes to 88. Even a Labour Party so denuded of members that it can muster fewer than 200 to choose a likely MP realises that the sloganeering of the Socialist Campaign Group is not the way forward.

What is worse for Brown, though, is that he must have realised last week that, not only is he stuck with Blairism, he is stuck with Blair. His predecessor is too clever to give him anything specific to complain about. Alastair Campbell edited his diaries to avoid giving offence by chronicling the endless struggle between the two heads of the dual premiership; Blair edits every word he utters – even in four and a half hours of interview with the BBC for last November's documentary – as he speaks.

It was unrealistic to imagine that someone who set new records for prime ministerial hyperactivism – and coming after Margaret Thatcher, that means something – would simply retire to spend more time practising on his classical guitar.

There are, though, those who have never heard of the rule, "If you want something done, ask the busiest person in the office". The New York Times editorial board wrote a pompous opinion article last week headed "Tony Blair's part-time job: Middle East peace". On the basis of Blair's accepting a really part-time job at Yale University, it whined pessimistically that he cannot be giving the Israel-Palestine issue his "full attention". And that was before he announced the "Breaking The Climate Deadlock" project in Tokyo. And it was before he launches the Blair Faith Foundation later this year.

It was also before Blair travelled last week to Jerusalem and Ramallah to talk to Ehud Olmert and Mahmoud Abbas. Of course, Blair's mission in the Middle East is virtually impossible. But it is to fall into the Blairite fallacy to imagine that, if only he devoted even more of his time and energy to it, it could be "sorted".

We were warned. In his last few years in office, Blair set out goals for himself, Britain and the world that were absurdly ambitious. Once he left office, as Jonathan Powell, his former chief of staff, said in an interview yesterday, Blair lost "the mechanism for saying no". In office he was defended by gatekeepers such as Anji Hunter, who Powell said used the phrase "FOFE, which was 'fob off for ever': you keep saying, 'Yes, of course, yes, of course', without ever fixing it".

Without those defences, said Powell, Blair committed himself to 462 days of engagements over the coming year.

But absurd ambition in pursuit of Middle East peace, the development of Africa or climate stability is admirable, however far it falls short, and there is no one else who could coax world leaders towards agreement so effectively.

As Gordon Brown – metaphorically speaking – watched the contrails of Blair's jet heading east from his train window, he may reflect that he has many virtues, but that kind of showing off is not one of them.