In the early hours of 5 July 2024, Keir Starmer arrived at Tate Modern in central London to celebrate Labour’s landslide election victory.As he prepared to address the throng of cheering activists, he was flanked by two people: his wife, Victoria, and his closest aide, Morgan McSweeney.A reluctant McSweeney, it was reported, was dragged on stage by the soon-to-be prime minister to a roar from the party’s foot soldiers.A few years previously, this moment had seemed impossible.Many believe that, without McSweeney, it would have been.

And yet, less than two years after that historic victory, McSweeney is out of a job, having resigned as the prime minister’s chief of staff on Sunday.His departure was triggered by revelations about the relationship between Jeffrey Epstein and Peter Mandelson, the man McSweeney lobbied to make ambassador to Washington.But it also comes after mounting pressure from Labour MPs, many of whom acknowledge the role McSweeney played in getting them elected but have also come to blame him for what they see as a toxic culture around the prime minister.Rob Ford, professor of political science at Manchester University, said: “McSweeney’s influence on the election strategy in 2024 was profound, for good and ill.“He deserves a lot of credit for the exceptional efficiency of the vote and the strong performance in marginal areas.

But he also played a big role in the neglect of safe Labour areas and a leadership which seemed to make a virtue of antagonising the progressives whose support Labour also needed.”Shortly before Christmas 2019, under the leadership of Jeremy Corbyn, Labour had suffered its worst election defeat since 1935.Voters in swathes of the party’s industrial heartlands in the north and the Midlands – from retired coalminers in County Durham to steelworkers in Scunthorpe – had battled the December elements to vote Tory for the first time.Labour seemed lost to the unelectable hard left and some predicted it would never win another general election.Yet less than five years later it had done just that.

Not only had the party clawed back most of its “red wall”, it had triumphed in a handful of seats that had never before had a Labour MP.McSweeney was credited as the brains behind one of the most staggering political comebacks in British history.The now disgraced Peter Mandelson, one of the architects of New Labour, summed up how the party’s moderates once felt about McSweeney when he said: “I don’t know who and how and when he was invented.But whoever it was, they will find their place in heaven.”McSweeney ensured that Corbynites were largely purged from lists of potential candidates, favouring instead moderates such as his wife, the former actor Imogen Walker, who became one of the 2024 intake of MPs.

He set out to spread the Labour vote as widely as possible to win the maximum number of seats.It worked: scores of Labour candidates squeaked over the line on the thinnest of margins.Despite its huge majority – Labour won by 174 seats, just five short of the 179-seat majority secured by Tony Blair in 1997 – it had gained power with just 33.7% of the vote.Ford described its victory as a “masterpiece of political Jenga”.

The rightwing press was less kind, calling it a “loveless landslide”.Loveless or otherwise, this historic victory led McSweeney to Downing Street.Just three months later, after a power struggle with the veteran civil servant Sue Gray, he became Starmer’s chief of staff.Despite his desire to eschew the spotlight, McSweeney will be remembered as one of the most powerful unelected political figures of recent years, joining the ranks of Alastair Campbell and Dominic Cummings.Yet it is McSweeney’s relationship with his mentor Mandelson that has been his undoing.

The pair have been close for years, and it was McSweeney who persuaded Starmer to appoint Mandelson as US ambassador despite knowing that he had continued a friendship with the sex offender Jeffrey Epstein after his conviction.Emails released by the US justice department appear to show that Mandelson sent Epstein confidential information about the UK’s response to the global financial crisis while serving as business secretary in Gordon Brown’s government.Mandelson has stood down from the Lords and is now at the centre of a police investigation.Starmer’s decision to trust McSweeney’s judgment on Mandelson – never a stranger to controversy – may yet herald his own downfall.The grandson of an IRA courier, McSweeney was born in Macroom, County Cork, in April 1977.

Although his parents canvassed for Ireland’s centre-right Fine Gael party, McSweeney showed little appetite for politics in his youth.His passion was sport: playing hurling and watching Liverpool FC.He moved to London as a teenager and worked on building sites before gaining a degree in politics and marketing from Middlesex University.He is said to have been inspired to join the Labour party by its role in negotiating the Good Friday agreement.He earned his political stripes fighting extremism across London, wresting control of Labour from its Trotskyite faction in Lambeth and seeing off challenges from the far-right BNP in Dagenham.

He won a reputation as a first-rate organiser but, in 2015, his campaign to install the Blairite MP Liz Kendall as the Labour party leader ended in crushing defeat: she got just 4.5% of the vote, losing to Corbyn.McSweeney spent most of the Corbyn years strategising on how to drag Labour back to the centre, spending several years at the helm of the moderate thinktank Labour Together.The strategy the group alighted on involved finding a candidate palatable to the left who would win the leadership and then steer it back to the centre.He alighted on Starmer, a former human rights lawyer and director of public prosecutions, as the candidate who could command the support of party members and the electorate.

When Corbyn resigned after the 2019 defeat, McSweeney oversaw Starmer’s leadership campaign – and his election to the role with 56% of the vote.The path to power was rocky.Starmer almost resigned after the disastrous byelection in Hartlepool in 2021, but that defeat prompted a change of tack from him and McSweeney.Jointly, the pair agree to go faster with their plan to change the Labour party, aggressively weeding out those they felt crossed a line, particularly when it came to tolerating antisemitic views.Corbyn himself was kicked out of the party, prompting a backlash from some on the party’s left.

Then in the run-up to the election, McSweeney oversaw a ruthless candidate selection process, which saw many MPs denied the chance to regain their seat after falling foul of the Labour leadership.Such was McSweeney’s influence on Starmer that one Labour source was quoted in the recent book Get In saying: “Keir’s not driving the train.He thinks he’s driving the train, but we’ve sat him at the front of the [driverless] DLR.”McSweeney proved to be a far less efficient government official than campaign mastermind, however.He was accused of presiding over a toxic culture in Downing Street, initiating vicious briefing wars against anyone who looked like a leadership challenge – including the health secretary, Wes Streeting, a former ally.

Some say he was so fixated on strategising for the 2029 election that he forgot his role was to help Starmer govern.During his tenure, Labour seemed unsure of what it stood for.It U-turned on everything from the two-child benefit limit to inheritance tax paid by farmers.The support McSweeney had fought so hard to cultivate quickly bled away and Labour MPs became mutinous, blaming him for what they see as an impending electoral disaster.Many in Labour circles blamed him for running what they saw as a “boys’ club” in No 10, which rode roughshod over the party’s own MPs and obsessed about keeping their allies in place and their enemies out.

The decision to appoint Mandelson in Washington was the ultimate example of that, his critics say.Now, the party is haemorrhaging support on the left and right.McSweeney was said to have been obsessed with winning back voters who are now backing Nigel Farage’s Reform UK, while paying little heed to its core left vote who are increasingly attracted by the Green party under its new leader, Zack Polanski.May’s local elections could sound the death knell for Starmer.It remains to be seen how he fares without his most trusted aide, who installed him in Downing Street in a seismic victory but leaves him as the most unpopular prime minister in history.