Gordon Brown made his pre-conference trip to America hoping that it would be a credibility-repairing, stature-boosting, authority-restoring platform to expound his global vision. He was instead forced to deny that he was going blind.

Rarely, if ever, can I recall a more humiliating television interview for a prime minister than being asked whether he was losing his sight. To make things worse, that question was part prompted by someone who was once a Cabinet colleague, though never a friend: Charles Clarke had earlier suggested that declining health may provide Brown with an excuse to dignify resignation from Number 10. By speculating openly about that, the former home secretary managed to destroy it as an option. The question about his sight was yet more mortifying because the interviewer was not British, but an American, Brian Williams of NBC's Nightly News.

The British prime minister will have gone on to the prestigious American news show expecting to get a more respectful hearing than he does from the British media pack who throw him into paroxysms of private rage. The United States would surely treat him with the deference due to 'the World Statesman of the Year' - for this he is in the estimation of a group called Conscience of the World. He instead found that travel was no refuge from his domestic griefs. When troubles at home pursue prime ministers abroad, it is one of the most infallible signs of political decay. As twilight fell on the premiership of John Major, he was pursued on a trip to Japan by sound bites from various obscure Tory backbenchers who were attacking him. During Tony Blair's closing chapters, he was chased from Australia to New Zealand and on to Indonesia by questions about how long he could cling on at Number 10.

When Brown was not being asked about his eyesight, he was harried about the non-resignation of the attorney-general. When it wasn't Patricia Scotland and her illegal housekeeper, it was Major-General Andrew Mackay who did quit after protesting about the treatment of troops in Afghanistan. Then there was 'the Great Snub'. Nicolas Sarkozy did not get a one-to-one with Barack Obama at the United Nations, not even a brief encounter in the kitchen, but the French media did not choose to interpret that as a crushing putdown.

For this, the prime minister had himself to blame by allowing his team to make five pleading requests to the White House. They were too "desperate", in the words of Mark Malloch Brown, the recently departed foreign office minister, to secure a Brown-Obama moment at the UN. This is what happens when a leader is trapped in a spiral of decay. Frantic efforts to contrive status boosters backfire and then leak out in a way that erodes you further. When the leaders travelled on from New York to Pittsburgh for the G20, there was plenty of hand-clasping, back-slapping and shoulder-to-shoulder time with the American president. Too late: the damage was done. Another avalanche of awful headlines crashed on his head.

Brown has not just lost the British media. The pack has turned on him with its unique brand of savagery, which it displays when it scents blood. The pursuit is whipped on by revenge. Brown acolytes have inflicted bruises on many Labour colleagues and quite a lot of journalists over the years. For some people, it is payback time and there is a lot of back pay. The ferocity with which the British press is hunting Brown is becoming reminiscent of Lord of the Flies when the other boys turn on Piggy and chase him to his death with the cry: "Kill the pig! Cut his throat! Kill the pig! Bash him in!"

So Brown goes into the Labour conference not as a man who has reasserted his credentials as a strong and visionary global leader - the prologue to Brighton that he hoped for. He appears before his party as a leader still imprisoned in a narrative of apparently remorseless and irretrievable decline.

The politico-media complex is pretty much unanimously convinced that this is his last conference as prime minister. At one level, this will not make the week in Brighton harder. It may even make it slightly easier. The Labour party, even in a demoralised state, has a natural tribal urge to rally publicly around their leader. They will give him a standing ovation even if he reads out the next day's weather forecast for Kirkcaldy.

To avoid a slide into irrelevance, Brown will have to do enough to keep his colleagues disciplined and his party sufficiently motivated to believe that they can still make a fight of it with the Tories. He will have to retrieve his dignity, buttress his authority and convince his party, the media and voters that it is not all over quite yet. There is one fragile advantage possessed by a leader who has been so comprehensively written off. He has the opportunity to surprise. To break out of the spiral of decay in which he is caught, he will have to produce a very big surprise in Brighton.