Like many Labour stories, Peter Mandelson’s and Morgan McSweeney’s both start at Lambeth council.

Mandelson was in his mid-20s. It was 1979, and he was a new councillor under the leadership of “Red” Ted Knight. He came to despise the local party, describing the Lambeth Labour party’s leadership as “contributing very little to the economic development of south London, instead politicising everything, attacking the police and the Tory government, and making the council go broke.”

Lambeth council was one of New Labour’s success stories, a successful recapture of local politics from the left. But by 2002, it had lost control of the council.

It was McSweeney – at a similar age – who fought to retake the council from the Conservatives and Liberal Democrats on behalf of his then boss Steve Reed. That was a partnership that was forged in local politics, which would later see both found Labour Together, in order to build a movement to retake the Labour party from Jeremy Corbyn.

On Tuesday, McSweeney will finally get his say at the foreign affairs select committee about the man who was described as his mentor and who he helped place in one of the most prestigious roles – US ambassador. It was a judgment call which cost him his role as the prime minister’s chief of staff.

For his part, allies of McSweeney say he will want to “correct the caricature” of himself in the press, including how close he was to Mandelson. Friends say he is irritated by the description of being Mandelson’s “protege” which they see as being Mandelson wishing to portray himself as far more vital to the project than he was.

But if that is the case, McSweeney will also have to explain exactly why it appeared so vital to appoint the twice-disgraced former Labour cabinet minister as US ambassador, including overriding convention and process.

Close observers of Mandelson and McSweeney often remark that the two men had significantly different politics in Labour terms. Mandelson’s emphasis on internationalism and social liberalism was rebuffed by the Starmerites who rejected this as the politics of a past era – and preferred a focus on community and security. Mandelson, for example, despised the Employment Rights Act and would brief against it to journalists.

But the pair had two mutual professional interests that overrode their political differences – firstly, destroying the Labour left and secondly, a ruthless pursuit of winning.

Mandelson and McSweeney had met sometime before McSweeney’s time in Lambeth. He was a junior administrator at Labour HQ, under Mandelson, and then in the attack and rebuttal unit. But they were reintroduced much later in their political careers by the Labour peer Roger Liddle – another veteran of Lambeth council – in around 2017.

They remained in very regular contact, by texts, calls, and visits to each other’s homes in Wiltshire and Lanarkshire, right through the darkest days of opposition after the loss of the Hartlepool byelection, until Mandelson’s ultimate sacking as ambassador.

When Keir Starmer became leader of the Labour party, with the help of McSweeney’s Labour Together, and McSweeney became his chief of staff, many of the New Labour old guard were privately dismissive of the project.

Many of them – including Tony Blair himself – had given up on the Labour party under Corbyn and had urged MPs to move to a new centrist, pro-European political party.

But in the early days, Mandelson was among the few who were genuinely interested in what Starmer and McSweeney were building.

Starmer himself was never friendly with Mandelson personally, though a leaked message to the Spectator made a warm reference to their political conversations.

Mandelson was scathing in private about the abilities of Starmer – and often rude about him in public, noting on the Times Radio podcast How to Win an Election that “Starmer needs to shed a few pounds and that would be an improvement.”

But he remained a feature despite his obvious contempt for Starmer – because of his closeness with McSweeney. Time and again his presence in the shadows worried those in Labour HQ. Simon Fletcher, one of the few staffers who worked for both Corbyn and Starmer, said he was alarmed to read in the Sunday Times of Mandelson’s growing influence in 2021, but that others in Starmer’s office played down his influence. Fletcher – who left shortly afterwards – grew to believe he had been misled.

Mandelson remained in close text contact with McSweeney and Starmer’s communications chief Matthew Doyle, especially as it grew closer to the election. The relationship was more strained with Labour Together by that stage, including a policy row with its leadership over some of its economic policies.

Sue Gray, Starmer’s then chief of staff, expressed concern at various points to allies about the influence that Mandelson seemed to be wielding in the background. “She knew he was after a job,” one former staffer said. “She took him off the shortlist to be ambassador,” one former minister said.

Though he was rarely in the office, Mandelson was on the regular roster of the Sunday supper club held at Liddle’s home in Kennington, which were also frequented by McSweeney and Doyle, as well as the now health secretary, Wes Streeting, and his partner Joe Dancey, who was a former Mandelson adviser. The dinners forged such a close-knit group that they continued through the Covid lockdowns and took place online.

It is telling about Starmer’s personal reservations about Mandelson that although he was persuaded of the need for a politician to take the role as US ambassador, he initially favoured the former chancellor George Osborne. There are few in Number 10 who would dispute that it was McSweeney’s personal endorsement that secured the job for Mandelson.

He now believes himself to have been personally betrayed by Mandelson’s failure to reveal the extent of his relations with the paedophile financier Jeffery Epstein – though documents revealed in the vetting process reveal a deep incuriosity.

Even in Washington, Mandelson was providing political advice to McSweeney until days before he was forced out, advising the elevation of Peter Kyle as business secretary and the removal or downgrading of more soft left figures like Ed Miliband, Lucy Powell and Jonathan Reynolds. Some ministers were aghast at this naked influence.

“He was in Number 10, masterminding the reshuffle, why the fuck is he doing that rather than working for us in Washington?” one said at the time.

But within days Mandelson was gone too, after new revelations in the Epstein files about the extent of his relationship with Epstein, including leaking government emails to him while he was serving under Gordon Brown during the financial crisis.

A bruised and furious Starmer has survived them all but his own future is in grave doubt. “He is absolutely furious at himself for having allowed the Mandelson appointment in the first place,” one friend said. “I think Morgan is too. I think they all rue the day they were ever convinced to let him back in.”