I don't keep much political ephemera. An Obama fridge magnet (designed to look like a paper bag of groceries - "Put food back on your table, vote Obama 2008"); a sardine can suggesting what tube travel would be like if Ken Livingstone was elected mayor; and a copy of the original New Labour pledge card (a smiling Tony Blair, teeth not yet perfect).

And a small pin badge - a few years old, a bit rusty around the edges. On it, the photograph of a smartly dressed middle aged man, slightly greying at the temples, the trace of a smile playing on his lips. "John 4 leader", it says.

I don't know how old it is. It could be nine years, could be six, because the John is John McDonnell who twice intended to run for the leadership of the Labour Party. The badge might also be quite rare, because on neither occasion - when Tony Blair stepped down in 2007 and then when Gordon Brown resigned in 2010 - was he able to follow through. Not enough of his fellow MPs declared they'd be willing to nominate him.

Image copyright PA Image caption John McDonnell disagreed with the direction Tony Blair was taking the party in 1997

Yet today, six months into the job of shadow chancellor, he's become one of the most influential figures in the Labour Party.

'Unwilling' to change

He's struck "absolutely the right note" over Google's tax agreement with HMRC, a former senior minister in the Labour government told me. Praise indeed, because for this veteran of the party's internal battles of the 1980s, Mr McDonnell's elevation is worrying because of his "rigidity".

Having dealt with Mr McDonnell in the late 1990s, when Tony Blair was Prime Minister, he says the shadow chancellor is "obsessed with positions adopted a long time ago" and "didn't want to listen, didn't want to change".

Tony Blair argued that Labour needed to change itself if it was ever to be allowed to change Britain. To Mr McDonnell and others on the party's left, that became an excuse to abandon too many of Labour's past positions.

Image copyright Getty Images Image caption GLC roots: John McDonnell (right) pictured in 1982 with Ken Livingstone (left) and other Greater London Council members

As the BBC's London political correspondent in the late 1990s, I vividly remember one of his attempts to get the Blair government to maintain one of those past positions. It was a relatively minor issue - voting arrangements in the City of London.

The local authority for the square mile, the Corporation of London (which has since been renamed the City of London Corporation), is the only one in the whole UK where businesses have votes in elections, whether or not those who run them live within the Corporation's boundaries.

The government agreed a compromise with the Corporation: in return for removing inconsistencies that favoured old City businesses at the expense of new ones, the City could keep the business vote, a decision which was in defiance of Labour's long-established policy. To New Labour this was pragmatism, a continuation of the "prawn cocktail offensive" under John Smith's leadership, to reassure a group traditionally sceptical about Labour.

To John McDonnell it was a betrayal of the basic political principle of one person one vote. For months he waged guerrilla war in the parliamentary trenches, sometimes alone, challenging clauses, forcing votes.

Likeable Corbyn

Why, then, was it Jeremy Corbyn, not John McDonnell, who stood for leader? Comrade Corbyn, the biography by Rosa Prince, which has just been published, offers an insight into this. To an extent, it was Buggins' turn. Mr McDonnell had tried and failed to get on the ballot paper twice before, while Diane Abbott had run in 2010.

Image copyright Getty Images Image caption Rosa Prince explains in her book why this time it was Corbyn not McDonnell

I think, though, some of it has to do with their contrasting characters. Unlike Mr McDonnell's previous bids, Jeremy Corbyn was able to reach the number of nominations, albeit at the last moment, because MPs like Dame Margaret Beckett nominated him even though they'd never dream of voting for him.

Put simply, despite being a serial rebel over three decades, he's likeable. As that ex-Labour minister put it to me, he's "easy to deal with". Even a former Conservative cabinet minister speaks with approval of the manner in which the Labour leader once challenged senior Iranian officials in a private meeting over abuse of human rights in their country.

Mr Corbyn will be 70 years old at the time of the next election, older by a few months than Michael Foot when he fought the 1983 general election.

Image caption Despite being a serial rebel, Jeremy Corbyn is said to be "easy to deal with"

Ticking boxes

Were he to decide to retire before then, John McDonnell would be an obvious choice for Corbynites who want to change Labour. In 2007 his abortive leadership pitch included leading "a real Labour government based upon the policies our supporters expect from us".

For a would-be Labour leader, his biography ticks a lot of boxes:

modest background, supported himself doing unskilled jobs whilst studying A-levels at night school

helped his first wife to run a children's home

was in charge of the huge budget of the old Greater London Council as it scrapped with Margaret Thatcher's government over its survival

won back from the Conservatives the constituency neighbouring Heathrow, turning it into a safe seat even as Labour's fortunes have waned

Yet that intransigence and what's been called a lack of warmth makes me wonder whether, even if he makes a success of being shadow chancellor, he could improve much on the small number of parliamentary colleagues who were willing to nominate him for leader before.

I was once told by a supporter of Denis Healey that the reason he never became leader of the Labour Party was his stubborn refusal to woo his fellow MPs. Principled of course, but even he later admitted he wished he had.

So I'll put the "John 4 leader" badge back in a drawer, but remember where it is - just in case.