The bookmakers, the pundits, the pollsters and - privately - many Labour MPs, all assumed Kim Leadbeater was headed for defeat in Batley and Spen.
Despite her extraordinary campaigns against loneliness and hate after the murder of her sister, the ex-MP Jo Cox, the 44-year-old's by-election bid was dismissed.
After humiliation in Hartlepool, it was seen as no match for the dual forces of Boris Johnson's "vaccine bounce" and George Galloway's divisive left-wing assault.
So when Labour won a skin-of-your-teeth victory - their majority is just 323 votes - the upset sent shockwaves through politics.
The PM has been denied another Labour-ousting northern MP for his burgeoning ranks on the Government benches.
And Labour's long-standing nemesis Mr Galloway cannot gloat about third place, despite winning some 8,265 votes.
Crucially for Mr Starmer, he is safe for now from threats to his leadership.
Party insiders crunching data and knocking on doors in the final days of the campaign - just as the Matt Hancock scandal exploded - felt a quiet confidence.
Lance Price, a former advisor to Tony Blair, was at the nerve centre of the highly-disciplined ground game as Labour struggled to hold the totemic constituency.
As it became clear the Health Secretary's Covid rule-breaking kiss with an aide - and subsequent resignation - was damaging the Tories, Labour was piling hundreds of activists into the area to win over disaffected Conservatives, while ruthlessly focusing on turning out its core vote.
"Clearly we turned up the energy in the last few days," said Mr Price.
"Ten days ago, I said to myself we have lost this and I was trying to keep my spirits up because we were determined to be proud of the campaign.
"But then over the weekend, we could feel the tide turn.
"Matt Hancock was certainly a factor. People who voted Tory in the past were pretty disgusted."
Meanwhile, Mr Johnson's party appeared to have been trying its luck with a new "predictive algorithm" rather than a traditional "get out the vote" drive based on data already gathered, one activist told the Mirror.
It meant Conservative activists potentially wasted time knocking on doors where support was effectively guessed at.
"I’d say of the un-pledged voters on our lists, the prediction was more accurate than not," the Tory grassroots member said.
"But we did get some doors slammed in our faces. A very mixed bag."
Mr Price said the sheer volume of activists in the seat was crucial for Labour, adding: "We don't have algorithms in the Labour Party.
"Keir said just now that Labour is coming home and that is about more than winning, it's about how we do politics.
"It's true to say we barely saw the Tory candidate during the campaign ... they were relying on Facebook and algorithms to win the election.
"Maybe sometimes you can do that but you shouldn't.
"You win an election by talking to people. We have spent the last five weeks just talking and talking and talking. Kim can talk for England. She can certainly talk for Yorkshire."
The efforts of new campaign chair Shabana Mahmood, who was drafted in to replace Deputy Leader Angela Rayner during a messy post-Hartlepool reshuffle, and Holly Lynch have been praised.
"It is always tight in Batley and Spen but every one of those 323 votes represent conversations we had with voters," said Ms Lynch. "We have knocked on doors relentlessly."
Ms Leadbeater's authenticity and deep connection with the seat has also been key.
"Nobody could say she has been parachuted in," said one insider.
Ms Leadbeater left the party when Jeremy Corbyn was leader and was persuaded by Mr Starmer to rejoin and fight the seat.
"She is the first of Keir's MPs - hard working, brilliant, local and not beholden to a faction," said one source.
Fears are growing on the left that the victory will be used to wage factional warfare.
Amid claims that some Corbyn supporters had helped Mr Galloway's efforts in an attempt to get rid of Mr Starmer, reports of members being expelled are already emerging.
But beyond the politics and internecine squabbles, Ms Leadbeater's win, five years after the death of her sister, represents something bigger.
During a rally in Cleakheaton, Mr Starmer condemned "misinformation, manipulation, lies and half-truths" during the by-election campaign - which saw Labour canvassers egged and assaulted, and Ms Leadbeater heckled in the street.
A fake Labour leaflet was circulated that declared war on "whiteness".
Mr Galloway will launch a legal challenge to the result, claiming he was defamed during the campaign.
"Ours was a values-based campaign," countered Mr Price.
"Kim is all about a better way of doing politics and a better way of public life, with integrity and honesty, not besmirching your opponents and going into the gutter and attacking them.
"This result is not just a shot in the arm for her and for the Labour Party and this constituency, but for politics and for the whole country.
"That is what Labour coming home really means."
Another Labour insider said: "I cannot stress enough the importance of this win is for Labour.
"Right now there is a tidal wave of emotion surging through all the campaigners which will undoubtedly carry the party forward.
"This was the worst by-election in a generation with smears, homophobia, intimidation and even violence and despite all that we won.
"Expect to see a more confident party showing real grit, fight and pride. It is a pivotal moment."
In her victory speech, Ms Leadbetter thanked her family, who she "would not have got through the last five years, never mind the last five weeks" and declared: "I am absolutely delighted that the people of Batley and Spen have rejected division and they voted for hope."
Perhaps the most poignant words of the last 24 hours, however, came from Jo and Kim's mother, Jean, who looking on during the rally at her newly-elected daughter said: "Jo would have been so proud of her.
"She was the one who always said to Kim 'go out, make the most of what you've got, do something'...and she did."
