Friday's award of the Nobel peace prize caps an astonishing journey of rebranding, redemption and self-rediscovery that transformed a defeated, lacklustre former vice president into the most famous environmentalist on the planet. It has also, inevitably, rekindled among the faithful the hope that, even at this late hour, Gore will enter the 2008 presidential race, and seek the office that has eluded him.

As launch-pad for a political candidacy, these last 10 months have exceeded the wildest strategist's dream: a hit film, an Oscar, and now the Nobel prize, lending him a moral authority that no potential rival in 2008 could possibly match. 'America and the earth need a hero right now, ' pleaded the Draft Gore campaign in a fullpage ad in The New York Times last week, urging him to run. Yet, this weekend, once again, he gave not the slightest indication that he will. After all he has been through, Al Gore, one suspects, knows better.

Defeat in an election that most felt he ought to have won with ease was a traumatic blow that Gore is still reluctant to discuss.

"It was difficult and you just have to make the best of it, " he told the Los Angeles Times last year. But the loss "did help me to focus on what was most important to carry me forward and right away this surfaced for me very powerfully."

It did not seem so at first. He grew a beard, spent time in Europe and taught a college course. Always a little fleshy, Gore also visibly put on weight. "If Al Gore slims down 25lb or 30lb, then watch out, " Donna Brazile, his 2000 campaign manager, said earlier this year of whether he was contemplating a run next year. As far as can be judged, most of those pounds are still there.

Oddly though, with politics lifted from his shoulders, Gore became a far more convincing politician. He spoke with a passion and urgency he had long lacked. In December 2002, he took himself out of the 2004 White House contest, saying he did not want the election to turn into a rematch with George Bush, that it was time for the Democrats to move on. By then, however, he was already an outspoken opponent of the all but certain war against Iraq, and a more eloquent critic of Republican excesses than he ever was as a candidate two years before.

In the meantime, private citizen Gore founded a cable TV network, joined the board of Apple and served as a senior adviser to Google. Most important, be hurled himself anew into the environmental issues that had fascinated him since his days at Harvard in the late 1960s, when Roger Revelle, one of his science teachers, warned students how greenhouse gas emissions would, if not curbed, devastate the earth.

In 1976, Gore was elected to the House of Representatives, where he organised the first congressional hearings on global warming. In the senate, where he represented Tennessee from 1984 until he was picked by Bill Clinton as his running mate in 1992, he pursued the issue. As vice president he helped broker the 1997 Kyoto protocol (never ratified by the US and repudiated by Bush).

But that activism pales beside the peripatetic Gore of the past few years, travelling the globe, lecturing, lobbying and starring in the documentary film An Inconvenient Truth. He also found time to write a book, The Assault on Reason, a devastating critique of the Bush administration, its trampling of the constitution, and of the US political system in general. All the while, honours and awards rained upon him, culminating in the Oscar and the Nobel prize. If ever Gore's hour had come, it was surely now.

Cryptically, he said on Friday: "I will be doing everything I can to try to understand how best to use the honour and recognition of this award as a way of speeding up the change in awareness and the change in urgency."

But every sign is that his supporters will be disappointed . . . this time around at least.

"I have no plans to do so, " he replies, mantralike, to the endlessly posed question of whether he will run. At a brief appearance in Palo Alto, California, he ignored it utterly. Now was the time to "elevate global consciousness" about the crisis, he told his audience. "I'm going back to work."

His cause is a higher one, the unwilling hero implies, that is better served by the public advocacy his celebrity makes possible, than by political office with its constraints and compromises. "The range of things we're talking about now will come to seem so small. [Climate change] is not a political issue but a moral and spiritual challenge."

But in the US, as nowhere else, once a candidate joins battle, morals and spirituality are part of the package. Were he to declare his candidacy, Gore would come to the game very late, less than three months before the first caucus votes, and $80m ( 65m) in fundraising behind Hillary Clinton and Barack Obama. Even so, a scenario can . . . just about . . . be constructed.

Suppose the cautious, ever-more centrist Clinton continues to lap the Democratic field and Obama and John Edwards fall even further behind . . . even as polls start to show Clinton faring less well in match-ups with Republican front-runner Rudy Giuliani. A "left-of-Hillary" space would open up.

Who better to fill it than the battle-tested, lionised Gore, now clad in the mantle of saviour of the planet? At least one poll in the vital first primary state of New Hampshire has suggested that he would vault over the former first lady.

Look more closely, however, and even larger obstacles emerge. A Gore-Clinton contest would split the party, and create conflicts of loyalty between their advisers. The Clinton marital psychodrama is moreover surely enough for voters. The media, bored with what seems the foregone conclusion of a Clinton victory, clamour for Gore to enter the race. The electorate might, however, suffer psychodrama overload that would rebound against the Democrats.

Moreover, a candidate is never more attractive than when he floats above the fray.

Once in the race, he might revert to bad old Gore, the leaden campaigner. The private Gore can be immensely engaging and funny.

Not so, however, in the public political arena, at least in 2000. "How do you tell Gore from his secret service detail?" the joke used to run. "They're the ones who smile."

Most important, Gore appears fulfilled in his current role, committed to the cause that gives him strength, and in whose service being ponderous, even boring, earns admiration, not mockery.

One last tantalising consideration: A Gore run may be unlikely in 2008. But if Republicans win next November, he could challenge in 2012, when he would be only 64. And even if a fellow Democrat wins, Gore would be a perfectly electable 68 in 2016.

Global warming will not go away. And nor will Al Gore.