Profile: Len McCluskey, Unite leader
Len McCluskey is general secretary of Unite, the UK's biggest union and the Labour Party's biggest financial donor.
That puts him at the centre of the ideological battle that pits leader Jeremy Corbyn and his legion of grassroots activists against large numbers of the party's MPs.
In the eyes of many observers, Unite's influence has played a significant role in securing Mr Corbyn's position over the past 15 months.
The Conservatives have styled him as "reckless" and a "union baron". They also described him as "unpatriotic" back when he declared the London 2012 Olympics a legitimate target for protest.
"I knew it was controversial," Mr McCluskey told the BBC's Hardtalk programme that year. "I'm angry, and I want people to be angry."
Elected leader of Unite in 2010, Mr McCluskey, now 66, is a firm believer in people power and showed political will from an early age.
He was born in north Liverpool in 1950, the son of a painter-decorator also named Len, and Peggy - a political inspiration to her son and once heard to remark that she would "rather bite off her arm than vote Tory".
He began his working life on the Liverpool docks - not as a docker but as a white-collar clerical worker, among whom there was no strong tradition of trade union organisation.
The then 19-year-old Mr McCluskey became a shop steward and helped unionise the white-collar staff, and then became leader of a union branch - the 5, 6, 7 branch.
The Casa, a social welfare centre and bar in Liverpool, is the modern hangout of this still fiercely loyal clique. Mr McCluskey launched his leadership of Unite there.
Tony Nelson, a docker from the 5,6,7 days who co-founded the Casa, told Radio 4's Profile in 2012 that from the start, Mr McCluskey had a wider political mission than just pay and conditions on the docks.
"Where we were different from a lot of other branches was that we were involved as much in pay and conditions as other disputes that were going on.
"It was lawful then to boycott cargoes that were coming into Liverpool and we were involved in the Chilean campaign against [military strongman General Augusto] Pinochet at the time, and Len was always at the forefront of these struggles."
Militancy and poetry
From the beginning, Mr McCluskey saw union politics as part of a wider solidarity. As the 1980s arrived - with him by now a full-time union officer - a more radical political vision was emerging in Liverpool.
The Militant Tendency was in the ascendancy - a left-wing, Trotskyist group that polarised the Labour Party and caught the national attention in their opposition to Margaret Thatcher's policies
"In those days, we were just coming off the backs of the docks being shut down, the factories being shut down. It was a major, major, major blow to the city of Liverpool," says Derek Hatton, Liverpool's former deputy council leader, who was eventually expelled from Labour over his membership of Militant.
"Everyone who was involved in that era couldn't help but be touched by that era."
Len McCluskey was never formally part of the Militant Tendency himself, although he supported many of its policies.
He also loves football - always Liverpool, never Everton - but his friends say he likes poetry or a night at the theatre too.
Labour's deputy leader, Tom Watson, once shared a house in London with Mr McCluskey. According to him: "He's very quietly intellectual. He's very well read, he's read nearly every poet on the planet.
"When you go out with him, if he gives you advice about life, it's usually in the words of WB Yeats or some of the Liverpool poets from the 1960s and 70s.
"His own language is very lyrical. He doesn't really weave that into his speeches or his political and industrial life, but to his friends he's quite a wise counsel, and there's usually a line of poetry to underpin his argument."
Radical inner circle
So does this more subtle style extend into his professional life as a trade union negotiator?
Alan Jones, industrial correspondent at the Press Association, covered Mr McCluskey's long-running negotiation with British Airways over cabin staff pay.
He argues that although Mr McCluskey is not afraid to be fiery or unpopular, he has risen to the top precisely because he is different from the intransigent union leaders of old - men like miners' leader Arthur Scargill.
"He is about as different from Arthur Scargill as you can get. I think he's more willing to cut a deal.
"I'm not saying he doesn't shout and rant, but since becoming general secretary of Unite, they haven't really been in that many really big disputes."
Mr McCluskey is not the archetypal trade unionist, at least not in appearance, but his friends say his politics are definitely hard-left.
He has expressed interest in direct political action with groups such as UK Uncut, and members of his inner circle are known to hold radical political views even further to the left.
"I think it tells us that Len McCluskey is not someone who is going to seek to be bound by the narrow confines of the Labour Party, but is going to look to reach outside of the Labour movement to build his coalition," says Dan Hodges, an ex-union worker and Labour Party commentator who is now a newspaper columnist.
"Whether that will ultimately be beneficial to him and his union, only time will tell."
As well as cultivating an external powerbase, how far has Mr McCluskey been trying to push the Labour Party to the left?
During the party's ill-fated spell of leadership by Ed Miliband, there was a perception that Mr McCluskey was pulling the strings to a certain extent.
Old friend Jim Mowatt says Mr McCluskey is unafraid to use the levers of power he has available to him to get both party and nation to dance to his tune - but does he have a bigger political vision for the UK, more than just representing the interests of the unions?
"Yes, and I would be disappointed if that wasn't his perspective," says Mr Mowatt.
"We want a Labour Party that respects the values of working people, that sits solidly behind lifting people's aspirations and helping people achieve their aspirations.
"The Labour Party in Parliament needs to look more like Labour voters."
Arguably this is what motivated Unite under Mr McCluskey to swing behind Mr Corbyn's leadership of the party.
And here is one final insight into the man. As it happens, Mr McCluskey has other passions you might not expect.
He loves karaoke, and he and Tom Watson have been known to sing Kings of Leon's Sex on Fire together.
But according to Mr Watson, Mr McCluskey may have sung "with gusto", but "he's a terrible singer".
This article was based on an edition of Profile broadcast in March 2012 on BBC Radio 4.