He was the political mastermind who oversaw Labour’s historic landslide victory nearly a year ago. Now Morgan McSweeney has become a lightning rod for criticism as the government faces its biggest ever parliamentary rebellion.

McSweeney, the soft-spoken chief of staff to Prime Minister Sir Keir Starmer, played a crucial role in purging left-wing Labour MPs, rebuilding the party as a centrist force and setting the government’s agenda.

Under the Irishman’s encouragement, Starmer swapped his previously left-wing clothing for a more patriotic, fiscally responsible, immigration-sceptic stance that helped deliver him into Number 10 with a massive majority.

But with Labour behind in opinion polls, the prime minister’s personal ratings slumping and angry backbenchers threatening to humiliate Starmer over his welfare cuts, MPs are starting to question whether McSweeney’s Downing Street operation is firing on all cylinders.

“Everyone is selling shares in Morgan,” said one Labour veteran. “People are starting to put their heads above the parapet and say maybe he’s not the Messiah after all.”

At the heart of the blame game is McSweeney’s belief that the Labour government needs to head off the threat of Nigel Farage’s Reform UK by leaning to the right on welfare, defence, aid and immigration.

Labour’s setback in the local elections in May — when it lost hundreds of seats to Reform — has emboldened McSweeney and other Starmer aides in their attempts to shore up Labour’s right flank.

They have polling data suggesting most of the public want stricter controls on welfare spending, and believe benefits reform is one way to claw back Labour’s six-point poll deficit behind Reform.

“People need to grow up and get serious . . . this is a problem with the country, not with Morgan McSweeney,” said one loyalist MP. “You can brief against staffers, slag off politicians, but the cost of welfare will still be unsustainable”.

That shift to a so-called Blue Labour stance has the backing of MPs in seats that backed Brexit and shifted to the Tories temporarily under Boris Johnson’s leadership — and are now vulnerable to Reform.

But many of the 100-plus Labour MPs set to rebel next week are more worried about the voters it is haemorrhaging to left-wing parties such as the Greens, and deeply uncomfortable about taking money away from some of the most vulnerable people in society.

Starmer’s so-called loveless landslide — which saw him propelled to power by a rejection of the ruling Conservatives rather than an enthusiastic endorsement of his agenda — makes Labour’s majority particularly vulnerable, people fear.

The “Marmite” view of McSweeney reflects a generational divide within the Parliamentary Labour party which one described as a “two tribes” situation.

They questioned why Number 10 was blindsided on Monday night when it emerged that more than 100 Labour MPs had signed an amendment aimed at blocking the welfare bill.

“Number 10 . . . should have seen this rebellion coming from outer space,” said one senior Labour MP. Instead, “they refused to listen, doubled down, refused to accept the criticism and just dug in. This is where we’ve ended up.”

In the early months of Starmer’s government, the prime minister’s first chief of staff Sue Gray was blamed for many of Downing Street’s problems. A former civil servant, she was in effect defenestrated by McSweeney in October.

“A lot of people drank the Kool-Aid that he was the fixer and Sue Gray was useless, but he can’t blame Sue for this one,” said the Labour veteran.

Another MP said McSweeney’s role in the government seemed to be to “shield” Starmer from uncomfortable truths.

“Other people in Number 10 were saying that he didn’t have the numbers for this and he wouldn’t get it through parliament. The chief whip has been warning them about this for months. But they had their fingers in their ears,” they said. “It’s extraordinarily arrogant and complacent.”

Others see in Number 10’s determinedness to press ahead with next week’s vote a sign of McSweeney’s desire to still confront Labour’s denuded left-wing. One MP from the 2024 intake said it seemed as though McSweeney was “spoiling for a fight” with the left of the party over the welfare reforms, which is a “very stupid thing to do”.

Allies of the chief of staff were involved in the selection process for candidates in last year’s general election, ruthlessly weeding out anyone seen as excessively left-wing. Yet the list of what is now 125 rebels is full of newcomers who made it through the net.

McSweeney’s supporters believe he will be able to shake off the growing animosity, insisting he has been a positive force within the new administration.

They say that — since the departure of Gray — the Irishman has helped Starmer get a grip on the Whitehall operation and improve morale. “They are much steadier, happier and more political than they were,” said one. 

“He backs staff unwaveringly, is in the trenches with us, and enables us to be political and keeps us sharp,” said one colleague. “Sue tried to knock the politics out of the operation, foolishly.”

He played a crucial role in recent trade negotiations with the EU and US, building relationships with respective advisers in Washington and Brussels: “They like, respect and trust him,” said one colleague.

McSweeney was also early to spot the risk of the winter fuel allowance cuts last summer which have since been largely reversed, according to allies.

But Pat McFadden, cabinet office minister and a highly influential figure in the Starmer government, has warned the prime minister of the danger of relentlessly targeting Reform-inclined voters, putting him at odds with McSweeney.

One ally of McFadden, who is a former aide to Tony Blair, said: “Pat believes we should have a big national message. It’s never a good idea to choose between one group of voters and another. Blue Labour is not Pat’s thing — it’s more Morgan’s thing.”

Luke Akehurst, the loyalist MP for North Durham, pointed out that in his constituency Labour plunged from 56 to four seats in the council elections — while Reform jumped from zero to 65.

“Labour is now clearly losing votes to Reform in its former heartlands and . . . the votes lost to the Greens and Lib Dems are more likely to be in seats that are safe enough that we can afford some loss of votes,” he argued recently. 

But now a phalanx of Labour MPs representing diverse and urban seats   fear that this shift to the right will end up alienating many of its natural supporters.

Polling data from YouGov suggests that far more voters who have deserted Labour since July have switched to the Lib Dems, Greens, Plaid Cymru or the SNP rather than Reform or the Conservatives.

Some left-leaning MPs accept the need for tougher policies on for example immigration, noting that net migration had been high in recent years.

But still many in the party recoiled at Starmer’s language about the dangers of Britain becoming an “island of strangers” in a May speech.

The divides within the party were on display during a gathering a few weeks ago of the Parliamentary Labour party.

“There were a slew of mostly London MPs with big majorities talking about progressive values and criticising the immigration policy,” said one pro-Starmer attendee.

“But there is also a caucus of younger MPs from the red wall and the growth group, who agree with some of those policies. You could really see two tribes in the room, and how it’s partly generational.”

One Labour strategist said: “Morgan is an easy lightning rod for people to blame for everything they don’t like about this government, which is not how government works in reality.”

A Number 10 aide said: “This government was elected to deliver change and it was never going to be easy. Our wins and challenges are owned by all of us.”