History is littered with the resignation letters of those who have tried to drive a wedge between Keir Starmer and Morgan McSweeney.

But that relationship is now facing its biggest test with the spotlight on the conduct of McSweeney as the former director of the Labour Together thinktank and whether there was any ill intent behind late declarations of more than £700,000 in donations.

The relationship between the prime minister and his chief of staff has often been called a marriage of convenience – and that has some merit.

Starmer rewarded the man who delivered him a decisive leadership victory and then a historic election majority. McSweeney identified Starmer as the most plausible candidate to defeat Corbynism – after the defeat of Jeremy Corbyn in 2019 – and crafted a platform.

But there is more to their partnership than expediency. Starmer has tried out other chiefs of staff over the years but none have suited him like McSweeney. His second chief of staff, Sam White, was a bad fit. His third, Sue Gray, tried to marginalise McSweeney and made enemies in the process, ultimately leading to her own exit.

Starmer has until recently implicitly trusted McSweeney’s strategic advice. There were whispers about his position after Starmer seemed to question that strategy in government in an interview with his biographer, Tom Baldwin. Starmer then made a dramatic show of defending his chief of staff to the whole cabinet, saying unequivocally that he believed they all owed their positions to the soft-spoken Irishman.

But unhappiness with the party’s rock-bottom poll ratings and a series of unforced errors means many of McSweeney’s internal critics are now sensing their chance to force a change of direction.

MPs blame McSweeney for the bruising battle over welfare and the hangover in government from the damaging missteps that Starmer made in his early handling of Israel’s assault on Gaza and a subsequent ceasefire vote.

And it is clear some of the classic McSweeney tactics have not worked in government. Giving no quarter to internal critics was disastrous in the run-up to the welfare vote, when even some of McSweeney’s closest friends and former colleagues went unheeded as they were sounding alarm the government was headed for defeat.

He has also borne the brunt of the criticism for the calamitous handling of the eventual sacking of Peter Mandelson, the former US ambassador.

The pair have long been close and there is deep suspicion that McSweeney equivocated when it came to Mandelson’s departure over deeply embarrassing emails with Jeffery Epstein. That is fiercely denied by No 10 sources but many MPs believe McSweeney should never have pushed through the coveted appointment for his mentor in the first place.

That fiasco was swiftly followed by the resignation of Paul Ovenden, his closest ally in Downing Street, over texts about Diane Abbott.

But this latest story to dominate the front pages – the undeclared donations to Labour Together – has the potential to be the most destructive because it concerns the very way that McSweeney and Starmer forged their relationship when the latter was mulling a leadership bid.

If there are more damaging donations revelations to come, that will taint Starmer himself and make the threat of a leadership challenge grow.

The new emails between McSweeney and Labour’s head of legal reveal the lawyer’s concerns about the donations. Gerald Shamash said there was “no easy way to explain how LT finds itself in this situation”.

Donors giving to Labour Together may well have been nervy given the vicious factionalism in Labour at the time. The organisation was also trying not to attract too much attention as a vehicle for a post-Corbyn leadership battle.

Vast sums being funnelled to a minor Labour caucus would certainly have done that. But they went broadly unnoticed, because they were not logged on time.

The Electoral Commission had decided Labour Together was a members’ organisation and thus needed to declare its donations. Since 2015, it has shape-shifted many times. It has been an internal Labour caucus, a thinktank, an incubator of talent and a leadership vehicle. Key figures have come and gone – once, the big names were Jon Cruddas, Lucy Powell and Lisa Nandy, many of whom are now out of favour.

It currently operates as a kind of arms-length policy brainstorming space intended to sketch out bold thinking on ideas such as ID cards and devolution, where young wonks can spitball about “project chainsaw” and plot their own paths to becoming MPs.

As a political force, it has seemed a little directionless with Labour in government. Its most recent director, Jonathan Ashworth, who narrowly lost his seat in July, quit after less than a year.

What it is mainly good at is raising money and funnelling it towards the party’s favourite sons and daughters. It was a major player in the 2024 election – and it gave almost £900,000 towards election campaigning, making donations towards the general election campaigns of 106 MPs. Only seven of those MPs had held their seats prior to the election. Its former director Josh Simons is now a minister in the Cabinet Office.

Should Labour Together under McSweeney be found to have committed more serious breaches, then the ripple affect might go much further than Starmer’s beleagured chief of staff. MPs will find themselves taking questions from opponents about why that money was accepted, when the carelessness of the body’s reporting has been known for many years.

There is potential for the scandal to spiral, even as No 10 insists there is nothing to see and that the penalty has already been paid.

The Conservatives and the rightwing press spot their biggest target yet. No one knows what Starmer looks like without his right-hand man. And it would remove the prime minister’s last remaining human shield.