The supporters of the Socialist Workers Party who gathered in Trafalgar Square on a bright sunny day at the end of March could not agree how to define the relationship between their organisation and the rally taking place around them. One seller of the weekly Socialist Worker, who was down from Sheffield for the day, told me that Unite Against Fascism was a “front” for the SWP, but the man working on the stall selling party literature was more cautious: “It’s not an SWP event,” he said. “We’re part of it. But it’s bigger than us.”

That was certainly true: UAF is an orga­nisation with many supporters, including many trade unions, and the demonstrators who had assembled at the statue of Nelson Mandela outside the Houses of Parliament had marched to Trafalgar Square beneath a wide array of banners. There were Socialist Worker placards saying “No to racism: blame Tories and bosses not migrants” but there were also banners of local branches of the National Union of Teachers (NUT) and the Labour Party. “Hugs not Thugs”, said one, and another, “Save Your Hate for the Daily Mail”. The speakers on the stage set up between the fountains in Trafalgar Square reflected the make-up of the crowd: Wayman Bennett, the joint secretary of UAF and a prominent figure in the SWP, was followed by Diane Abbott and Christine Blower, the general secretary of the NUT.

The speakers were interspersed with bands, evoking memories of UAF’s predecessor the Anti-Nazi League, and the great days of Rock Against Racism in the late 1970s and early 1980s. The SWP has always sought to “punch above its weight”, as the saying goes, by attempting to co-ordinate a broad constituency in support of a cause. But at the moment it has a particular interest in surrounding itself with respectable figures, and in directing attention towards its anti-fascist campaigns, because it is seeking to repair the damage caused by a scandal that has played out over the past 18 months.

In 2010, one of its leading members, who has always been referred to as “Comrade Delta”, was accused of sexually assaulting a young female “comrade”, and the party’s attempt to deal with the matter via a “disputes committee” composed largely of his colleagues has provoked anger and derision. Three further allegations of rape prompted claims that sexual abuse was “endemic” within the organisation.

Yet it was the suggestion that the leadership had protected one of its own, and persuaded hundreds of members to collude in a cover-up, that convinced many people it was irredeemably corrupt.

In March, the University of London Union, which used to let rooms to the SWP for its annual conference on Marxism, changed its constitution to allow its officers to ban the party from the premises and accused it of being a “rape-apologist organisation which prides itself in creating an unsafe space for young women”. The attacks are not only verbal: recently, SWP stalls have been overturned at student demonstrations, and its activists harassed and abused.

The man working on the stall at Trafalgar Square articulated the defiant view that the leadership has taken throughout the affair: “We’re not going anywhere,” he said. “If anyone thinks we are, they’re crazy.”

Yet such loyalty is increasingly rare: hundreds of former members have left the party, many with scornful parting words for their former comrades. “If I had died last year I should have died happy to have been a party member,” wrote a long-standing member, Ian Birchall, in his resignation letter. “Unfortunately, the events of the last year have changed everything.” Birchall’s remark that he had never seen a “crisis remotely comparable to the one we are now going through” carries some weight. He had been a member for 50 years and wrote a biography of Tony Cliff, the revered Trotskyist activist who set it up.

Cliff was born Ygael Gluckstein in Palestine in 1917. He was the son of a Zionist building contractor, although Paul Foot – the campaigning journalist and long-standing SWP member – said he was “speedily converted out of Zionism by observing the treatment of Arab children”. In 1947 he came to Britain, where he changed his name, and established the Socialist Review Group, which became the International Socialists (IS) in the early 1960s and then the SWP in 1977. It defines itself as “a voluntary organisation of individuals who understand the need to organise collectively to fight for the socialist transformation of society”.

The transformation required is absolute, “for the present system cannot be patched up”, and it will be achieved only “through the self-activity and self-emancipation of the working class”. Tony Cliff said that “the emancipation of the working class is the act of the working class” and the concept is often expressed by the slogan “Socialism from below”. Christopher Hitchens, who was an early member of IS, said that the result of a “revolution from below” would be that “those who worked and struggled and produced would be the ruling class”.

Hitchens went on to become features editor of Socialist Worker, the party’s newspaper, and book reviews editor of International Socialism, its theoretical journal, but when he was a student at Oxford in 1967, his local branch of IS had no more than a dozen members. “For a long time, these groups remained tiny,” Foot wrote, after Cliff’s death in 2000. Yet the SWP became the dominant force on the far left in the late 1980s, in the lead-up to the dissolution in 1991 of the Communist Party of Great Britain.

The end of the cold war had strengthened the SWP. It seemed to bear out Cliff’s view that the Soviet Union had never been a socialist society, but a “state capitalist” one, which “people on the left had no reason to defend”, as David Renton, another member who left this year, said to me. “Cliff toured the country, addressing rallies, saying I was right,” he recalled when I met him at his house on an estate near the Caledonian Road in north London.

Renton is an Old Etonian and the nephew of a former Tory chief whip. By the time he joined the SWP in 1991, he had become used to living in “a perpetual civil war” with his family and contemporaries at school, his resignation letter said. He had been involved in other organisations on the far left, but he was drawn to the SWP because he felt it was playing a positive role in the upheavals of the time, and because of its approach to revolutionary politics. “They were serious about the project, and the years it would take, while not making the compromises with capitalism that would mean giving up before you started,” he told me.

David Renton said that the SWP believed it was the natural home for people to the left of Labour but it became apparent during the 1990s that there was a “size threshold” it couldn’t pass. “The history of the SWP in the next 20 years is watching a series of attempts to take this image of themselves as a mass political party and give it legs,” he said.

Richard Seymour – the author of a critical account of Hitchens’s journey from revolutionary socialist to advocate of the war on terror – joined the party in 1998. “The situation politically wasn’t offering much hope,” he told me, “but people had lots of anecdotes about past experiences. They were saying we were nearing the beginning of a mass movement, and when the anti-capitalist movement kicked off around ’99, and the anti-war movement after 9/11, we had a sense that they were probably correct.”

The attempt to set up an organisation to exploit the anti-globalisation campaigns failed, but the party had more success with Stop the War, which was launched after the 11 September 2001 attacks, and reached its apogee at the mass rally in London to demonstrate against the impending invasion of Iraq. Few of the people who went on the march on 15 February 2003, myself included, would have known it was organised by the SWP, and even fewer joined the party as a result. But the scale of the protest offered a glimpse of the influence to which the SWP aspired.

It attempted to capitalise on its success by forming an alliance with the Respect Party, whose public face was the MP George Galloway. Galloway won the parliamentary seat of Bethnal Green and Bow in London for Respect in 2005 and later became MP for Bradford West, but the alliance with the SWP collapsed in 2008. Respect’s national chair at the time, Linda Smith, blamed the SWP’s “sectarianism” and “control-freak methods”, while the SWP said Galloway and his allies were moving to the right.

The SWP had gained nothing from the venture, the journalist Paul Anderson writes, except a “few recruits . . . and a lot of ridicule for cosying up to barmy reactionary Islamists”. One of its periodic bouts of infighting ensued: John Rees and Lindsey German – “the two leading figures most responsible for the Islamist turn”, in Anderson’s phrase – were expelled, and a new national secretary, who would come to be known to the wider public as Comrade Delta, was appointed.

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The first complaint against Comrade Delta was made in 2010. A woman who was referred to as “Comrade W” accused him of sexually harassing her, and he stepped down as national secretary while remaining part of the party’s leadership: its central committee, or CC. The party was told about the allegations at its conference in 2011.

Alex Callinicos – professor of European studies at King’s College London and grandson of Richard Lyon-Dalberg-Acton, the 2nd Baron Acton – introduced the session at which they were discussed. As the SWP’s international secretary and the editor of International Socialism, Callinicos is the party’s chief theorist, but according to Richard Seymour he was also its “main pugilist” throughout the Delta affair. His speech has been described as “a euphemistic triumph”. “At no point did Callinicos talk of sexual harassment or sexual assault,” a former member wrote. “He made it sound like there had been a lover’s tiff,” David Renton says. “He gave the impression it was a relatively minor row, and said we have dealt with it because we have slightly demoted this figure.”

Comrade Delta spoke next: he told the delegates that if they “knew the very worst he was accused of, they would gasp at how empty the story was”. Other leading figures spoke on his behalf, and Renton says the delegates responded “to every signal that the misconduct was of the mildest character possible by chanting, ‘The workers united will never be defeated,’ and gave [Delta] a standing ovation.”

Rosie Warren, a student at Sheffield University who joined the party during the student occupations of 2010, said it was a very uncomfortable event: those who were not applauding were either as confused as she was, or “some combination of disgusted and appalled”.

Charlie Kimber, the party’s new national secretary, maintains that the standing ovation was provoked, not by the dismissal of the allegations of sexual harassment, but by another attack on Delta. “I very much regret the two became intertwined,” he told me.

The assurances that the affair was “a bit of a misunderstanding” and that “both Delta and the female comrade wished to put it all behind them” soon proved false. Comrade W was not satisfied with the result of the original complaint; in fact, she came to the conclusion that she had understated her case. She left the SWP in the autumn of 2010 because she felt she could not remain a member while Delta was on the central committee, but she rejoined a year later and in September 2012 she accused him of rape.

Even then, many people in the party still “didn’t want to hear it”, Richard Seymour says. There were pragmatic reasons for that. Despite a subscription-paying membership of no more than 2,000, the SWP employs 50 or 60 people full-time at its headquarters in Vauxhall, south London – and the national secretary decides who gets the jobs. What’s more, many people liked Comrade Delta and his strategy for the party. “He said we don’t need big united fronts and all the rest of it: the workers and the trade unions are going to start fighting back against austerity, and we have to help that struggle along,” Seymour recalls. “A large chunk of the party had great sympathy with this.”

The younger members were not so easily placated. The generational divide had personal and political dimensions: David Renton told me that “almost all the young full-timers took against Delta” because they didn’t like him.

Others found themselves at odds with the party’s old-fashioned attitude to feminism, which it associated with “a separatism that doesn’t really persist, particularly on campuses”, Rosie Warren says. “The feminism we’d come across was focused largely on harassment and assault, and getting angry at victim-blaming narratives,” she says. “So the knee-jerk reaction we saw in the party when everything came out was completely alien to us.”

Soldier of some revolution from below: Christopher Hitchens’s first job was at Socialist Worker. Photo: Muir Vidler for the New Statesman, 2010

The party’s decision to investigate the allegation internally, through its disputes committee, rather than referring it to the police, is the most remarkable aspect of the affair: it has astonished people outside the SWP, and some within it, too. “What right does the party have to organise its very own ‘kangaroo court’ investigation and judgment over such serious allegations against a leading member?” wrote the former Socialist Worker journalist Tom Walker in his resignation letter. “None whatsoever.”

David Renton, who is also a barrister and has dealt with cases of rape and sexual harassment, believes that it didn’t occur to the disputes committee to suggest that the woman should go to the police – as one of its members later said, the committee had “no faith in the bourgeois court system to deliver justice”.

Comrade W’s reasons for not reporting the case to the police are less clear, but Renton suggests she may have had two concerns: as well as the understandable fear that the police would treat her case insensitively, she may have believed that their priority would be to secure a conviction against the leader of a revolutionary party – an attitude, he adds, that stems from an overestimation of the SWP’s significance. “People on the left often do this,” Renton says, citing Julian Assange’s belief that the rape charges against him must be politically motivated because he is “the world’s number-one bad guy”. In other words, she may have been trying to protect the organisation from what she saw as a “predatory man” who should not be in a leadership position, and from state scrutiny.

Regardless of what her motives were, Comrade W was “doubly betrayed”, says another former member called Linda Rodgers. She came to the SWP because she trusted it, and it should have told her it wasn’t competent to investigate. “Would the DC [disputes committee] have investigated a murder?” Rodgers wrote. “I would guess not, but then what does that say about the level of seriousness with which the CC and DC treat rape?”

Kimber maintains that because the complainant did not want to go to the police, they had no choice but to investigate themselves. Yet the decision left the disputes committee “hopelessly out of its depth”, David Renton says. None of its members had relevant experience, nor did they not seek advice from party members who were lawyers. “I’m gobsmacked that no one ever said

to the SWP, ‘Look, if you take statements, you’re collecting criminal evidence.’ ”

Published accounts of the hearing, which was held over two days in October 2012, exposed even more egregious flaws: Comrade Delta was supplied with details of the complainant’s case weeks in advance but she was not allowed to see his evidence beforehand, and the committee members – who included colleagues of Delta’s, old and new – asked her questions about her drinking habits and sexual past. Comrade W left the room in tears, saying that they thought she was a “slut who asked for it”.

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By the time the disputes committee presented its report to the SWP’s annual national conference at Hammersmith Town Hall on 4-6 January 2013, the revolt against the party’s handling of the case had begun: four members, who became known as the Facebook Four, had been expelled for discussing the case on social media and two dissenting factions had emerged, each with the support of 50 or 60 members. “The party was split in two,” Rosie Warren says. “My organiser was desperately trying to get each half of our district just to sit together.”

The DC told the conference that it had reached a unanimous verdict: Comrade Delta had not raped Comrade W. It also found that he was not guilty of being “sexually abusive or harassing”, though not unanimously: the chair of the committee said he had decided “that while sexual harassment was still not proven, it was likely that it had occurred”. He also felt that Delta’s conduct “fell short” of what “one should expect of a CC member”.

The complainant was not allowed to speak, though she had wanted to, but other people spoke on her behalf: one asked the conference to reject the report because of the “serious failings in the way the hearing was conducted” and another said that W felt “completely betrayed” by the way she had been treated since the hearing. The conference was also told that a second complaint of sexual harassment had been made against Delta which the committee had not investigated. “It was all beyond belief,” Rosie Warren says. “I wasn’t the only one who cried after that session, from fury as well as despair.”

The delegates were given no good reason to approve the report, beyond that the people on the panel were long-standing members with good reputations. “I couldn’t believe those voting in favour of the report had been sat in the same room as me,” Warren says. “I couldn’t believe they were people I had respected, taken leadership from – I couldn’t believe that we were even in the same organisation. I couldn’t believe the injustice.”

The motion passed by the narrowest of margins – 231 for and 209 against, with 18 abstentions. Yet the leadership did not treat the result as a warning, or a cause for reflection: critics say it was still not too late to moderate its approach, but instead it imposed its authority by insisting that Comrade Delta had been vindicated and that anyone who did not accept the vote should leave the party.

News of the disputed report soon spread: a transcript of the debate on the DC’s report appeared on the Socialist Unity website on 7 January and people started asking what was happening. Three days later, Tom Walker resigned from the SWP and from his job on Socialist Worker, saying he did not believe that “anyone sensible” would ever join the party again.

“That was the beginning,” Richard Seymour says. Soon, the “bourgeois media” picked up the story: Laurie Penny wrote an article for the New Statesman website and the Daily Mail joined in.

Pressure came from outside the organisation, as well as within: union organisers wrote an open letter asking the CC to reconsider its approach to the case, and journalists and academics, including Ilan Pappé and Owen Jones, said they would not speak at events organised by the SWP. Linda Rodgers called on all the members of both the CC and the DC to resign, and China Miéville, the science-fiction and fantasy writer who stood for parliament in 2001 for the Socialist Alliance, the SWP’s electoral coalition, declared that “the fight for the soul of the SWP is now on”.

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The argument was partly about the nature of the SWP’s internal processes. It operates what it calls “democratic centralism”, which means that policies are debated during the three months running up to conference, and voted on at conference. Once ratified, all members are required to support them. In effect, argument is silenced for nine months of the year, and even the conference debates are severely curtailed. According to Rosie Warren, a member of the central committee would introduce each session with an overarching description of the year’s events, after which lowlier members would report successes in individual workplaces or campaigns. At the next session, delegates would be handed a summary of the discussion and invited to agree with it by vote. “It always struck me as really bizarre because there was nothing to vote on,” she says. “It was just a description of the session.” It is hardly surprising that many members saw the Comrade Delta case as not only disturbing in itself, but illustrative of a “deep democratic deficit” within the party.

Its broader culture was also called into question. “When you treat human beings as disposable objects in the name of la causa, when appropriation of activists’ labour and good will is the norm, when exploitation of your own side goes unchallenged, sexual abuse is one probable outcome,” wrote Anna Chen, who worked unpaid on various SWP press campaigns, including Stop the War. She believed the SWP’s habit of “ripping off their activists for wages, thieving their intellectual efforts and claiming credit for their successes” had initiated a pattern of “diminishing regard for their members”, which had led to the point “where even someone’s body is no longer their own”.

The party’s hierarchical structure and its culture of “loyalty beyond logic” concentrated power in the hands of the central committee at the Vauxhall headquarters. Yet the leadership had no intention of “opening up the party’s structures”, as its first response to the debate made plain. Towards the end of January, Alex Callinicos published a long article in Socialist Review, the party’s monthly magazine, which examined the necessity of “deepening and updating Marx’s critique of political economy” and referred to the Delta affair, in passing, as a “difficult disciplinary case”, significant in so far as it prompted “a minority” to dismiss “democratically reached conference decisions” and, hence, undermine democratic centralism.

What the dissenters were arguing for, he wrote, was “a different model involving a much looser and weaker leadership, internal debate that continually reopens decisions already made, and permanent factions”. Such changes would make the SWP “smaller and less effective”. Defending the handling of the Delta case was synonymous with defending the party’s revolutionary purpose.

In March, the leadership conceded to demands for a second conference to re-examine the allegations, but only on the most unconciliatory of terms. “Let us be clear that this comrade has been found guilty of nothing,” said the pre-conference bulletin. That was true – Comrade Delta has never been formally charged, let alone tried or convicted, and is entitled to the presumption of innocence like everyone else. Yet it was not his guilt or innocence that was in question, but the way the party had dealt with the complaint.

The leadership refused to acknowledge the criticism. It said the March conference was to “reaffirm the decisions” of the January conference and, sure enough, the “opposition got smashed”, Richard Seymour says, because people “who had never been seen in the organisation turned out to vote”. China Miéville had said that the conference would be “a last chance to save the party from disgrace”, and when it was over, he, Seymour, Rosie Warren and many others resigned.

David Renton stayed on because he wanted to see if they could take the complaint any further. He had met the second complainant, Comrade X, in February, and “was absolutely convinced that in every single thing she said she was telling the truth”. In the summer, the disputes committee concluded that Delta had a case to answer – but he would not have to answer it because he had left the party: the inves­tigation would be reinstated only if he should choose to rejoin. “Essentially, they admitted that the second complaint was probably true,” Renton says. “Which obviously cast a light backwards on the first complaint as well.”

In March, before the special conference, another member had told the Guardian she had been raped: she said that the problem was “a systemic thing” and that the SWP was a “dangerous environment to be in”. In October, a fourth woman revealed that she had also made a complaint. She said she had been raped in December 2012. She reported the case at the end of January 2013, after the handling of the Comrade W case had provoked outrage within the party, and yet she was treated in exactly the same way. The two women from the DC who interviewed her asked, “What effect would you say drink and drugs had on you that night?” and encouraged her to drop the complaint. A pattern had become apparent, the woman maintained: “. . . the Socialist Workers Party is a group that is sexist, full of bullies, and above all will cover up rape to protect its male members and reputation.”

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Not surprisingly, Charlie Kimber dismisses the allegation. “It is wholly untrue,” he told me. “If I believed it for a moment then I would not be the party’s national secretary – or a member of the party.” It is partly because the SWP takes the oppression of women seriously, he added, that the case was so painful for it. He said it could hardly be accused of attempting a “cover-up”, as the case provoked non-stop debate for the best part of a year and prompted the party to elect an independent body to review its disputes procedures. “Did the Lib Dems act in this way over allegations of harassment?” he asked. “Has the Labour Party?”

The new disputes procedure was announced in December, at the party’s third conference in a year. The code corrected some of the flaws made apparent in the Comrade Delta case, and the CC also issued a partial apology to the complainants. “We are sorry for the suffering caused to them by the structural flaws in our disputes procedures . . .” Kimber wrote. Even that fell far short of the full apology and whole-hearted invitation to self-examination that its critics wanted. But David Renton realised that the leadership had gone as far as it could. “If they had admitted that they got things wrong, and genuinely apologised to these two women, they would have had to stand down, and completely overhaul the organisation. In a sense, that was the story of the last year – why a bunch of us said things and why, beyond a certain point, the organisation refused to listen. Because if they had listened, they would have had to switch the organisation off.”

Yet many people have maintained that the leadership’s attempts to save the party had the opposite effect. “You think you won in Hammersmith,” wrote a member called Richard Atkinson in his resignation letter that March. “You didn’t: you lost. For all the foot-stamping and cheering you lost, comprehensively and probably irrevocably.”

David Renton and 165 other people left in January to form a new group called rs21 (Revolutionary Socialism in the 21st century) and he believes the SWP has been left with no more than 200 active members. Richard Seymour says its rump of “worker-ist activists” is “brain-dead, unpleasant and thuggish” – and destined to become more so. “It is toxic,” he says. “It’s doomed.”

Rosie Warren’s verdict is even more damning: she says the only thing left for the leadership to do is to issue a full apology, and then “declare that anything that was ever good about the SWP has been utterly destroyed, and pack up and go home”.

Charlie Kimber says the party is “far from doomed”, though he concedes that the left cannot afford any more splits. Unfortunately, its propensity for internecine conflict seems undiminished. The International Socialist Network, which Richard Seymour, China Miéville and others set up after leaving the SWP, lasted less than a year before disintegrating over an online argument about a sexual practice called “race play”. Seymour now believes it will take a generation to reconstruct the left, and might not happen at all. But the implosion of the SWP has given it a starting point, at least. David Renton believes it will have to begin with an appraisal of the failings of the party to which he belonged for most of his adult life. “Our mistakes were so awful that anyone trying to rebuild the left is going to have to say, ‘We are not at all like them.’ ”

Edward Platt is a New Statesman contributing writer. His most recent book is “The City of Abraham: History, Myth and Memory – a Journey through Hebron” (Picador, £9.99)