He's not as well known to the public as many leading politicians, but he wields considerable power within Labour. So what does union boss Len McCluskey want from the general election?
Len McCluskey's influence is far-reaching in Labour circles.
His Unite union has donated more than £11m to the party since Jeremy Corbyn became leader.
Close allies from his union have served in the leader of the opposition's office and at party HQ - including Jennie Formby, Labour's general secretary.
Members of his union are also influential on the party's ruling National Executive Committee.
Mr McCluskey refused entreaties from Labour's deputy leader Tom Watson to broker a timetable for Mr Corbyn's departure when eight out of 10 of the party's MPs voted no confidence in their leader after the 2016 referendum.
This burnished his credentials as a staunch supporter of the current leader.
He will be a key figure when Labour holds its "Clause 5" meeting this weekend - a behind-closed-doors gathering of senior shadow ministers, union leaders and members of the party's national executive to decide the contents of the election manifesto.
Mr McCluskey often - but not always - gets his way.
He is a skilled negotiator who is prepared to make compromises - or, if you like, cut deals - to maintain his influence.
For example, he is no fan of plans to hold a second EU referendum. Yet in order to keep most of the Labour-backing unions on the same page, he advocated a position that - albeit tweaked - will effectively be the manifesto policy on Brexit.
He has accepted the need to offer a referendum with Remain on the ballot - but only if a Labour government still negotiates a Leave option of its own.
Labour wouldn't decide officially whether to back Leave or Remain until it held a special conference to discuss its new deal.
Mr McCluskey has already called for shadow ministers to refrain from setting out how they would personally campaign in a referendum during this election.
So far, he tells me in an interview, he has been pleased that Labour's top team is exercising "discipline" on this issue. And he is confident that "they will continue to hold as disciplined a line as they can without making themselves seem stupid".
But the BBC has seen campaign literature from two junior shadow ministers who are explicitly suggesting a vote for them is a vote for Remain.
Mr McCluskey says he has made an appeal to shadow ministers and the party policy is clear: "Labour is not a Remain party. Labour is a party which says we will respect the 2016 referendum and we will negotiate a credible exit from the European union."
More than 130 candidates have signed a "Remain pledge" - they are not waiting to make an assessment of any future Labour-negotiated deal.
Alex Sobel is one candidate whose leaflets are adorned with the EU flag.
He told me: "We have to have two options on the ballot... but I'll always want to stay in the EU as the most beneficial thing for the UK.
"I am asked on the doorstep where I stand on this, and it would be dishonest to say I hadn't made up my mind."
Mr McCluskey is far more relaxed about what backbenchers put out in their campaign literature, recognising that Labour voters and the wider nation are divided on Brexit: "Many MPs come from high Remain areas and high Leave constituencies and they need to respond to the local position."
But his main focus is trying to get the Labour campaign to focus on issues beyond Brexit, highlighting policies that can appeal to potential voters in Leave and Remain areas alike.
He cites recent initiatives big and small - from free prescription charges in England to greater consumer rights and a new "green deal" on the environment.
"We can concentrate on issues where Labour is streets ahead and appeal to ordinary working people."
And in Leave areas in particular - what he calls "the forgotten towns and cities which gave the political elite, as they saw it, a slap in the face" - he wants to emphasise policies which "reinvigorate de-industrialised towns".
These include raising the minimum wage, eliminating zero-hours contracts and investing more in infrastructure.
He said he could also reassure voters in these areas that any future referendum will not be an attempt to remain in the EU by any means - that there will be a genuine choice.
But there is one issue where some party strategists have been concerned.
The 2017 Labour manifesto stated that "free movement" - giving EU citizens the right to work and seek employment here, and vice-versa - would end with Brexit.
But at this year's conference, delegates backed a motion which called for free movement to be "maintained and extended".
The Conservatives have claimed that this means immigration would increase under Labour - a message which might not be welcome in Leave areas in particular.
This weekend Labour has a big decision to make - should a conference policy be converted into a firm manifesto commitment for government?
In our interview, Mr McCluskey dismisses as "total nonsense" a newspaper report which suggested he had warned Mr Corbyn to take a "tough line on free movement of workers".
There has been strong pressure from the grassroots Left to maintain free movement.
A leading member of the anti-Brexit group Another Europe is Possible, Michael Chessum, has argued that the policy would be popular with voters who might otherwise be tempted towards the Lib Dems and Greens - and would underline a radical transformative agenda from a future Labour government.
Rather than take a "tough line" on the issue, it sounds as if Mr McCluskey is working on a form of words that might avoid a clash with the grassroots while offering reassurance to those who were worried that their wages are being undercut by the free movement of cheap labour.
He suggested people would be less worried either about free movement or - in the case of Brexit - continuing labour mobility if protections were put in place. If Labour frames the issue as an employment rather than an immigration issue, perhaps a political circle could be squared.
"Labour's policy will be to protect all workers - migrant workers as well as British workers," Mr McCluskey declares.
"It will be done with labour market regulations. It won't stop the free movement of labour.
"It will effectively make certain that greedy bosses, agency companies, are not abusing working people."
Another major decision is whether the party's conference policy on nationalising the "big six" energy companies should become a manifesto commitment.
Mr McCLuskey says he has spoken to Labour's energy team. "An element of public control" is required, he says, as "foreign companies are ripping off British citizens". He adds: "A debate on how the energy of our nation is controlled is central."
So watch out for the wording when the manifesto - always the product of politics by committee - is unveiled next week.
There are tricky internal issues for Labour to negotiate. But whatever emerges is likely to be even more radical than in 2017.
Mr McCluskey believes distinctive policies which break out of the Brexit straitjacket are not just the way to transcend the divisions which the referendum created or highlighted - but will also offer Labour its best hope of electoral success.
We won't have long to wait until the voters give their verdict.