As graduates of the country’s best university, most former Cambridge students neither seek nor expect much in the way of public sympathy. Last weekend, however, the frontrunner in the Labour leadership contest, Andy Burnham, attempted to elicit a little.

Describing his journey from a Merseyside comprehensive to Cambridge as the thing which ‘brought me into politics’, he told of his bewilderment when, as a prospective English student, he was asked at his interview, ‘Do you see a parallel between The Canterbury Tales and modern package holidays?’ He was, he said, ‘still pondering what the question meant when I arrived at Warrington station six hours later and when the rejection letter dropped through the door’. The setback was temporary — he eventually won a place — but he says he never lost ‘the sense of being an interloper’.

This is the key to understanding the man most likely to be the next Labour leader. He may come across as a machine politician, but he feels very much like an outsider, battling the elite of his own party. The former health secretary’s path to Cambridge was not, it is true, smoothed by a private education. And it is easy to see how, on arriving there, a Catholic Liverpudlian from a working-class background, albeit a comfortable one, might have felt an initial sense of unease. But as one senior Labour figure puts it, from the time Burnham graduated from Cambridge, he has been the beneficiary of ‘the ultimate conveyor-belt career’: he glided into a job working as a researcher to Tessa Jowell and then a post as special adviser to culture secretary Chris Smith. By 31, he had a safe seat in parliament, and he stayed on the fast track: home secretary David Blunkett’s PPS aged 33, in the Cabinet by 37.

In those days the Labour orthodoxy was Blairite, and so, it appeared, was Burnham. As a junior member of Tony Blair’s government, he appeared to be an avid reformer, with little sympathy for union objections to the growth of private provision in the NHS. But his views changed, along with the prevailing mood. As health secretary in the dying months of Gordon Brown’s government — with defeat looking inevitable, and an eye on wooing the powerful Unison union in a future leadership contest — he began to soften his rhetoric on public service reform. Nonetheless, when the contest came, Burnham refused to join Ed Miliband’s march to the left: he strenuously defended the government of which he had been a member, his support for the Iraq war and identity cards, and his belief that Labour shouldn’t match the Tory promise to protect health spending.

But on Saturday 25 September 2010, Burnham’s graceful, if not effortless, ascent suffered a staggering blow. Before the TV cameras and the delegates to Labour’s annual conference, he learned that he had been pushed into fourth place — beaten not just by the Miliband brothers, but by Ed Balls, a figure reviled as much in some Labour quarters as he is by the Conservatives. Burnham had only narrowly avoided finishing last behind Diane Abbott, a hard-left candidate who was only on the ballot paper because David Miliband had let some of his supporters nominate her.

Five years on, he is the last man standing from the 2010 contest: David Miliband exiled to America, Ed Miliband’s dreams of becoming prime minister obliterated, Ed Balls ejected from parliament and Diane Abbott distracted by her latest fruitless publicity-seeking endeavour: a run for London mayor. But the Burnham we now see is a rather different candidate from the one that Labour rejected so decisively in 2010. In the past five years he has undergone a complete reinvention: from one of Tony Blair’s rising stars to the toast of union baron Len McCluskey, from New Labour insider to the darling of the grass roots.

It would be easy to paint Burnham’s change of direction — his striking of left-wing poses, his sometimes mawkish sentimentality about the NHS, and his cosiness with the unions — as purely the result of ambition. Not so. Its roots lie in the friendships, rivalries and jealousies of the tightly knit group of Blairites to which he clung at the outset of his Westminster career.

‘All politics is personal for Andy,’ says a friend of his, recalling the rupture which occurred when Burnham, who had been expected to back David Miliband, decided to run for the leadership in 2010. Friends and allies urged him not to. Not only was it David’s turn, they warned; he would embarrass himself by standing. The advice, no doubt well-intentioned and ultimately prescient, enraged him, compounding long-festering insecurities and suspicions. Just as Burnham continued to feel an imposter at Cambridge, so too, argues a friend, he always seemed to feel that he remained outside New Labour’s charmed inner circle. Others who had trod the same path from working-class backgrounds, such as Alan Milburn, David Blunkett, John Reid, Hazel Blears and Alan Johnson, did not seem to share the sentiment.

Perhaps Burnham’s feelings were driven by the fact that he wasn’t a policy wonk like the Milibands and like his closest political friend, the former work and pensions secretary James Purnell. He’d long suspected that Purnell was the favourite of the New Labour leadership. Despite his Cambridge degree and his undeniable intelligence, Burnham never displayed the intellectual self-confidence of some of his colleagues.

To his ears, the objections to him running were simply snobbery, confirmation that there was an elite within the New Labour elite, of which he was not a part. He is a stubborn man, and the counsel that he couldn’t and shouldn’t stand simply encouraged him to do so. There is, says a friend, an element of ‘the man’s not for turning’ to his character.

Despite the determination with which he entered it, Burnham hated the 2010 contest. He found the atmosphere between the Milibands and their rival camps toxic. His team suspected that those around Balls — desperate to avoid coming fourth — were briefing against him. Burnham couldn’t understand why people he considered more than colleagues — hadn’t he been to their weddings and attended their children’s christenings? — had failed to support him. Burnham believes that disloyalty is one of politics’ greatest sins. He was genuinely sickened and angered by the manner in which the Brownites plotted to unseat Blair in 2006. Eventually — and not so much because he was promoted, friends say, as because Gordon Brown backed his campaign for the victims of the Hillsborough stadium disaster — he came to exhibit a similarly fierce loyalty to the new occupant of Downing Street.

In 2010, Burnham experienced first-hand the conflict between personal and political loyalties. ‘Where was James?’ he asked plaintively after the contest, referring to Purnell’s backing for David Miliband. Did he really expect such loyalty in politics? Purnell might have asked a similar question in June 2009, when Burnham refused to join his bid to topple Brown by resigning from the Cabinet. Supporters of David Miliband, bruised by his narrow loss, blamed Burnham for refusing to say publicly that he would give his crucial second preference vote to their candidate.

In the aftermath of defeat, and having grown to dislike Ed Miliband, Burnham considered quitting the shadow cabinet altogether. That would not have been an altogether surprising course for this deeply emotional man who, friends say, often eschews political calculation. Instead, suggests a party insider, Burnham chose to digest the lessons of his defeat. He needed to toughen up and never again let himself be outmanoeuvred by the likes of Ed Miliband — who he felt had run a shameless campaign, careless of past positions and record.

After five years of making himself acceptable to the left of his party, Burnham has spent the first weeks of this contest tacking back to the centre — praising business and wealth-creation and hinting at support for Tory welfare cuts — and presenting himself as a ‘unity’ candidate: he’s tapped the support of Blair loyalist Charlie Falconer, recruited centrist MPs like Rachel Reeves, Michael Dugher and Dan Jarvis, and won the backing of the hard-left MP Ian Lavery. It is a coalition so wide that it may well give him the leadership. If it does and he becomes leader of Her Majesty’s Opposition, we can expect Andy Burnham to remain, at least in his own mind, an outsider.