Jeremy Corbyn has fought two leadership contests, in 2015 and again last year. And even though his name is not on the ballot paper, he has ended up in the middle of a third one being fought now. The battle to head the most powerful union in Britain, which would normally be confined to issues such as pay, pensions and job security, has been turned into a proxy war over Corbyn.

Len McCluskey, general secretary of Unite since 2011, is up for re-election and the ballot closes on 19 April. Most union elections go largely unnoticed but this one is high-profile because McCluskey is one of Corbyn’s biggest backers. Without him, the Labour leader would be left seriously exposed.

In a wide-ranging, hour-long interview with the Observer in a hotel near Unite’s headquarters in central London, McCluskey says he was “cautiously optimistic” that he will see off his main opponent, Gerard Coyne, who he describes as being supported by a cabal of Labour MPs and grandees intent on ousting Corbyn.

“Coyne’s campaign is run by a cabal of West Midlands MPs who are trying to abuse Unite’s democracy by fighting a proxy war against Corbyn. Now that is regrettable and I believe it will be rejected by our members. This campaign, this election, is about Unite. It has nothing to do with Corbyn.”

It has been, he says, “a shameful campaign full of lies, innuendoes and smears”. It is strong stuff, but McCluskey is keen to speak not only about the challenges facing Labour, including the falling-out with his former friend, Labour’s deputy leader Tom Watson, but a host of industrial disputes across the country the union is involved in, his general despair over the low-pay economy, his fears over Brexit and his views on antisemitism.

McCluskey, 66, was brought up in a working-class community in Liverpool. Although his public persona is that of a bruiser, one-to-one he is often affable. Words that can look brutal in print can be softened with a laugh.

He quotes a song: “I was born in Liverpool, down by the docks, my religion was Catholic, my occupation hard knocks.” He failed the 11-plus but passed a 13-plus for late developers, leaving school with three A-levels. He started as a docker in the Canada Dock, a 10-minute walk from his home. He became a ship’s planner, drawing up maps of where cargo was placed so it could be easily found at each destination.

He was politicised partly by the spirit of the 1960s, partly by the solidarity he shared with the dockers and partly by the community he was brought up in, influenced by his mother and father, not activists but solid Labour supporters. He became a shop steward at 19, a regional officer in the Transport and General Workers’ Union at 28 and 11 years later took a national job, moving to London.

Unions, in spite of the decades-long fall in membership, are for him as relevant as ever, central to the economy and the fight for better conditions. “This so-called, much-heralded, flexible market that even New Labour and Tony Blair talked about, this is a cancer within our society: zero hours, low pay, bogus self-employment, agency labour everywhere. The so-called gig economy. This is all part of a race-to-the-bottom culture which my union has been at the forefront of fighting against for many years now.”

He is especially irate over the treatment of public sector workers: the people who bind a community together – the teachers, refuse collectors, carers, health workers and the many others. “We have seen this horrific attack on Westminster Bridge, a terrorist attack, and we have seen the staff of St Thomas’ hospital on the bridge, flooding out of the hospital with total disregard to their own safety. These people’s professionalism and heroism was just unbelievable,” he says.

“And yet just a week later they were told they were going to get a measly, lousy, 1% pay rise, which effectively means a cut in living standards. What kind of society do we have that does not recognise the decent people we have in our public service.”

He lists a series of disputes he is engaged in, perhaps to answer criticism from Coyne that he spends too much time on Labour and not enough on members’ interests or, maybe, just to demonstrate the range of industrial actions going on every day that go largely unreported. On Monday, he heads to Babcock Marine to discuss a dispute at the nuclear base at Faslane on the Clyde, expressing outrage at the prospect of military personnel being used to replace striking civilian workers. Other disputes on his list involve the energy company Ineos at its plants in Grangemouth, Scotland, where he says the owner is threatening to tear up bargaining agreements. He mentions workers at BMW in dispute over pensions and another row over pensions at the Atomic Weapons Establishment.

He expresses concern about the impact of Brexit on investment, listing talks with the major car manufacturers which have told him they are already making plans for development elsewhere in Europe.

Against this background, where is Labour? He disputes the idea that there is no effective Labour opposition to the government, citing party figures saying Labour has forced 30 U-turns on the government, and praises Keir Starmer, the shadow secretary of state for exiting the European Union, for a “fantastic” performance over Brexit.

He has said that Corbyn had 15 months to rectify the problems facing Labour, which was interpreted as a deadline. What did McCluskey mean? Corbyn was being slaughtered day in and day out in the media, he says, “aided and abetted, of course, by a group of Labour rightwing MPs, only a small group, and grandees but they are powerful enough”.

He expresses hope that Corbyn can break through and cites policies he said were popular such as free school meals, which was announced last week. What is needed is the party to unify as the electorate do not like division.

He adds: “If the party remains divided, then we are in deep trouble and that is the whole issue about a constant review. I did not say Jeremy has only got 15 months. I said that knowing the type of man he is he will himself want to constantly review things to see how we are performing, how the Labour party is breaking through on its policies. Of course, that is something that any serious politician and leader would want to do all the time.”

If Corbyn thought he was not turning things around, would he just go? “I don’t want to speculate on that. I really don’t. I am saying he should be given a fair chance, him and John McDonnell, who I have been very impressed with.”

Labour seems unable to go from one week to the next without some self-inflicted crisis. Last week it was former MP and London mayor Ken Livingstone being suspended over remarks about Hitler and Zionism. McCluskey describes Livingstone’s outburst as “extraordinary” and supports the way Corbyn dealt with it by asking the party’s ruling national executive to review it. But with regard to antisemitism in the Labour party, McCluskey said he has been a member of the party for 45 years and never experienced any form of antisemitism.

And the prospects for the 2020 general election? “The truth is that on a personal basis I think that whoever was leader of the Labour party after 2015 would find it an unbelievable task to win the 2020 election given we have lost Scotland and it is not coming back any time soon and there are going to be boundary changes which are going to impact adversely on Labour,” he says.

“But I don’t think the Tories will win the next election. They might be the largest party but I don’t think they will be able to form a government. And I do see a prospect of a Labour government, not in coalition, but with a Queen’s speech that is supported by the SNP and Liberal Democrats and obviously what negotiations go on at the time would be down to the Labour party.”