James Naughtie looks slightly crumpled and rubs his eyes. He has been up since three this morning, at his desk since four and on air since six, grilling ministers who counter his questions about the possible effects of biological attack with windy evasions. It is a thankless task performed with boundless enthusiasm. At the Today programme debriefing in editor Rod Liddle's cramped office afterwards, Naughtie is full of sharp observations about what worked and what didn't. Tired but still combative, he seems a little hurt that Ed Stourton has been sent to Washington rather than him, and suggests he head for Pakistan to report from the potential war zone, a one-man ask force.
I hadn't been looking forward to meeting Naughtie. I find his radio presence, with the orotund questions, the interjections, the feeble attempts at humour, irritating, though since he is forever winning awards I am clearly in a minority. But in person he is more engaging than I had anticipated - articulate, direct, not in the least bit pompous.
Interviewer has turned interviewee because he has just published a book about the relationship between Tony Blair and Gordon Brown, billed as "the intimate story of a political marriage". Fourth Estate paid him a reported advance of £300,000 for the book, the Mail on Sunday ran the first extract of its serialisation over five pages, and then the world fell in. It is a masterpiece. Of bad timing. But, of course, in terms of the victims of the tragedy the book is the least significant, and Naughtie is not weeping over it. In any case, he says, in a transformed world the Tony and Gordon show will still be running. "There's no angle that you can look at politics and government in this country that doesn't pass through them. They are the prism that sheds light on everything."
The book has had a frosty reception (David Hare in the Observer called it "a dog"), but it offers a pacy account of a 20-year relationship that has remoulded the party and might one day do the same to the country. Naughtie, an opera lover, has tried to shape the narrative in almost Verdian terms - as the story of two characters at the centre of great events but not quite in control of their destinies. It may be more Ernani than Don Carlos, but at least he has tried to write an accessible book that might appeal to a politically apathetic age.
"I have deliberately written about character rather than event," says Naughtie. "I wanted to focus on where Blair and Brown came from, what makes them tick, how this relationship works. The fascination to me is the duality. If they were completely bonded and agreed with each other all the time, or if on the other hand they couldn't stand each other, then it wouldn't be terribly interesting. What's fascinating is the twin-track relationship - one track of which is intense, intuitive feeling for each other, and alongside it a track which has a lot of irritation, incomprehension, mystification."
Most of the pre-publication interest centred on the supposed Blair-Brown deal cooked up at the Granita restaurant in Islington, and there has been disappointment that Naughtie could find no smoking bun to prove that Blair double-crossed his old friend. In fact, says Naughtie, there was no formal deal, merely a recognition by Brown that in terms of the leadership the game was up.
"Brown knew much earlier than was generally assumed that it was all over," he says. "There were a lot of people around him who didn't, but he had been preparing himself for a couple of weeks, so the basis on which they spoke was that they both knew the score. Brown had made the decision not to stand for the leadership the day before." He already knew he could not win, and thus was not in any position to make demands at Granita.
So has Brown's chance of being PM gone? Naughtie is sure that Blair will see out a second term and thinks that, five years on, Brown may be yesterday's man. "He still wants to be prime minister and is consumed by his ambition," says Naughtie. "But times move on and fashions change. If Blair were to fight and win a third time, by 2005 Brown would have been chancellor for getting on for 10 years. Who knows how the political landscape will look? I can't imagine Brown serving in a cabinet under another prime minister and he may well look for a big international job instead. It's all or nothing - that's the conundrum." Naughtie thinks it is less than 50-50 that Brown will inherit, and names David Blunkett and Charles Clarke as the men to watch.
As for the Blair-Brown relationship, he believes that it is sure to end in tears. "However it ends, it's going to be a cataclysm. People in the Treasury are contemptuous of some people at No 10, and many people in No 10 are contemptuous of Brown. They are two rival camps. Some of those at No 10 were even saying before the election that Blair should sack Brown."
The dramatic thrust of Naughtie's story is that Blair and Brown - the smooth lawyer and the hard-edged activist, the New Labour chameleon and the old Labour champion - complemented each other perfectly in remaking their party and winning power, but no longer know how to relate to each other. "There's a residual closeness," he says, "but a lot of the old fire has cooled. The strains are pretty continuous and reverberate through the government. People say that Blair spends a third of his time dealing with Brown, sorting out a problem, clearing something, or soothing a dispute. Thatcher and Lawson were nothing like that."
It is easy for Hare to mock the book's "laptop prose"; I was more inclined to express surprise that it had been written at all. Naughtie, who is 50, usually does Today four times a week, presents opera broadcasts and has a monthly book programme, as well as making lucrative appearances on the conference circuit. "It's been quite tough," he admits, "and I've been guilty of burning the candle at both ends." He lives in Kew with his wife Ellie and three children; he may even see them occasionally. "There have been stresses and strains, but my wife and children have been great," he says.
Have there, as rumoured, also been stresses and strains with his co-presenter John Humphrys? Neither, after all, is a shrinking violet. Are they the Blair and Brown of broadcasting? "John and I are very close colleagues," insists Naughtie. "You can't come in and sit next to somebody at four in the morning without getting to know them pretty well. Presenters are in competition to a degree, but that healthy competition is no more voracious or destructive than it would be on a newspaper where everyone wants to get the front-page lead. It's nothing like as corrosive as people outside like to fantasise."
Naughtie can speak about newspaper life with authority. He was political correspondent of the Scotsman and then of this paper (where his lunching and expense claims were legendary), before becoming presenter of The World At One in 1988 and then of Today in 1994. He has no regrets about the move into broadcasting. "This is journalism in the raw and it has tremendous excitement. You're in a very privileged position being able to talk to people at an extremely intimate time of the day - insinuating yourself into their living room, their bathroom, their car. That's a great feeling."
The "war" may have damaged the prospects of his book, but it has given a fresh urgency to Naughtie the newshound. Now, what's the best way to the Khyber Pass? (And don't let Captain Humphrys know.)
• The Rivals is published by Fourth Estate, price £16.99. You can read the first chapter at theguardian.com/books.
