Alan Johnson is rueful about rumours at Westminster that he is being groomed as the Anybody But Brown candidate for the Labour leadership when Tony Blair steps down. "Don't put money on it," he advises. "It's science fiction - but nice science fiction." Less convincingly, he adds: "I got rid of my leadership tendencies in the Communication Workers Union. I've got it out of my system. I wanted to get to the top there. I don't really want to get to the top here. It's just fantasy."
But nice fantasy, all the same, among the knots of Labour MPs sitting around in the bars with nothing better to do than speculate about the post-Blair era. Johnson presses so many of the right buttons: a lefty in his early years in the labour movement, a capable and articulate leader of a loyal union, a combative performer in the Commons, a safe pair of departmental hands and a hot-gospeller for party modernisation. He was the only major union leader to champion the abolition of Clause Four.
To this raft of attributes may be added a well-honed killer instinct. The way the Work and Pensions Secretary recently threw the Child Support Agency boss, Doug Smith, to the wolves on live television in front of a Commons select committee excited murmurs of admiration. Could this man be the answer to the prayers of skittish maidens on the back benches who cannot stomach the prospect of Gordon Brown in No 10? The answer is "probably no"; but "never say never" remains a wise adage in politics, and Johnson did not say the N-word.
That the idea could be seriously discussed is a measure of how far Johnson has come since entering the House in unusual circumstances in 1997. Considering how voluble he is on every other topic, but especially pop music, Johnson is curiously coy about how he came to be an MP. He lays bare the painful memories of childhood and the political militancy of his early years, but refuses to talk about how he suddenly made the transition from union baron to Blairite foot soldier at Westminster.
The myth-makers claim that he was drafted in at the last minute when the virtually invisible MP for Hull West, Stuart Randall, went to the Lords. In fact, Johnson had been approached by Blair with the offer of a safe seat some months before, when the Major government seemed close to collapse. When the call finally came, Johnson was driving to his mother-in-law's birthday celebrations. He knew nothing about Hull (and still does not know the old tyke joke that "the Humber is the arsehole of Britain, and Hull is eight miles up it"), but he didn't hesitate. The snappily dressed sarf London boy sailed in with a majority of more than 15,000.
Johnson's origins were humble. His father, a painter and decorator, abandoned the family when Alan was only eight. When his mother, a cleaner, died later, he and his elder sister were almost put into Barnardo's homes, but fell instead into the care of Mr Pepper, a child welfare officer. His 15-year-old sister so impressed the authorities that the young orphans were given a council flat in Battersea, and Johnson attended Sloane Grammar School across the river in Chelsea. He left at 15, without qualifications. "They were pleased to see the back of me," he recalls. "My sister used to do her best, but I wasn't interested in anything." Oh, except English, economics and history - not bad for a teenage rock band player in the wild mid-1960s. He stacked shelves at Tesco, before walking out because he wasn't allowed a lunch break.
So the union militant was born. Johnson, already married with two kids, became a postman in Slough at 18, attracted to the job by promises of lucrative overtime. Dorneywood, the ministerial grace and favour house, was on his round. Just before the 1971 national Post Office strike, his third child was born, and he "took on" the Heath government while on paternity leave. He joined the Labour Party shortly afterwards.
As a union branch official, he gained the reputation of being a militant. But, in one of those giveaway moments, Johnson still gets riled about being called an ultra-lefty. "I wasn't a Trot," he insists. "I was more CPGB [Communist Party of Great Britain]. I did consider myself to be a Marxist - I read more chapters of Das Kapital than Harold Wilson." He also read Jack London and Gramsci and admired the showy Clydeside communist Jimmy Reid - not, be it noted, the more pivotal figure of Jimmy Airlie.
Johnson became a full-time union official in 1987. A series of mergers led to him becoming general secretary of the former Union of Communication Workers in 1993, representing a hard-left executive from whose politics he was growing ever more distant. But he cut more of a dash than his rivals. When the government decided to privatise the Post Office, Johnson turned the tables by hiring Lord (Tim) Bell, the Tories' public relations guru, to help scupper the plans. Royal Mail remains in public hands, just about.
By then, Johnson was on Labour's National Executive and, after the 1997 election, the newly elected MP was appointed parliamentary private secretary to Dawn Primarolo - hardly a match made in heaven, but a rung on the ministerial ladder that took him to Trade and Industry in 1999 and then on to the Education department as higher education minister. He proved a cunning, persuasive and, above all, successful campaigner in the battle to get the back benches to support "top up" university tuition fees. The measure was carried by just five votes, and beforehand the whips were so unsure of victory that Blair discussed the terms of the motion of confidence that would have followed such a major defeat. The orphaned grammar school boy without an O-level to his name cut a more convincing figure among old Labour stalwarts than Charles Clarke, his hectoring, public-school-educated boss. "I was part of the charm offensive with Charles Clarke," Johnson said recently. "I did the charming and he was offensive."
He was rewarded by a grateful Blair with Work and Pensions when Andrew Smith, shortly before he was pushed, did his spectacular self-defenestration from the cabinet. There was not a long queue for the job. The Child Support Agency was heading for virtual meltdown. Adair Turner's report on pensions was due, revealing a £50bn shortfall in the nation's pensions provision, with 12 million people not saving enough for their retirement.
Johnson maintains, however: "There is no pensions crisis." When the subject was debated in the Commons, he crushed the Tories' David Willetts, quoting Labour's achievement of lifting two million pensioners out of abject poverty. It was in the days following this performance that talk of his possible leadership candidacy began to emerge.
At 54, in good health despite his snow-white hair, in a rock-solid seat, he could be at the top table for another decade. He is the first ex-union general secretary since Frank Cousins in 1964 to make it to the cabinet - and he is already more successful than that disastrous precedent. But how far could he go? He seems no more likely to become Chancellor than the manager of Queens Park Rangers - which, he says, is the job he really wants. He has shown little interest in foreign affairs. Nor could he ever match David Blunkett's natural unpleasantness as Home Secretary. But he is the archetypal safe pair of hands and it is possible to see him at the head of Health or Education, or at Trade and Industry, where he might one day have to preside over the privatisation of Royal Mail. He would make an ideal leader of the Commons, or a canny Chief Whip if Labour's majority were to fall perilously low after next year's election.
As for the party leadership, Johnson, like Brown - but unlike Alan Milburn, who is more commonly talked about as the Blairite candidate - has the happy knack of gathering MPs around him. I spoke to him on the night after he had been talking to the Yorkshire group of Labour members. He rhapsodises about the government's record on employment: the lowest jobless figures for 34 years, the highest employment rate in the G7 countries, 51 per cent of disabled people at work, and so on. He hymns the dignity of work, and insists that the scourge of unemployment has been beaten not by a miracle, but by government policies - the New Deal, welfare-to-work, childcare and tax credits. This is the language the troops like to hear. It is their language, and he speaks it like a native because he is a native.
In that sense, Johnson sounds more like Gordon Brown than his ostensible hero Tony Blair. He is currently immersed in Roy Jenkins's biography of Churchill, perhaps picking up a few tips on late-developing political careers. He might also ponder Jenkins's remark, in his book on Gladstone, that "nearly all prime ministers are dissatisfied with their successors, perhaps even more so if they are from their own party". It would be rich indeed if Blair were to be disappointed by his protege in this way.