Sir Keir Starmer warned during the election campaign of the need for "tough decisions", but carefully avoided setting out where the axe would fall.
Now it's clear who will be losing out - starting with most pensioners losing winter fuel payments worth up to £300 - unease is bubbling under the surface.
Politics live: Number 10 not 'softening' winter fuel payment cut
There is no doubt the government will win Tuesday's vote as they have a huge majority of 174.
But the number of abstentions - or MPs who cannot face voting for it - especially if they number dozens, will test the prime minister's authority and signal whether his backbenchers have the stomach for more of these cuts.
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Tap hereOver the summer, Labour MPs have seen their inboxes fill up with pensioners and their families angry that those who rely on the payments fear they will face a cold winter in hardship.
The benefit will be restricted, Chancellor Rachel Reeves announced in July, to those who claim pension credit, and no longer given to the 10 million people aged over 66 who don't.
She told MPs at a meeting tonight that it was a difficult decision, and she "wasn't immune to the arguments against it", but that sticking to it was a question of economic credibility.
Government sources claimed she had won the argument that "'no one likes it, but we have to do it".
Pensioners, she said, could blame the Conservatives for leaving a financial black hole.
The problem is that 880,000 pensioners who are eligible for this top-up do not claim it, so they will lose out despite being the poorest - including some on just £13,000 a year.
The government has run a campaign aimed at increasing the uptake, but the payments will go straight away.
Campaigners - pensioners have vocal campaign groups on their side - also say the million or so people just above the threshold will also struggle.
Dozens of Labour MPs are weighing up whether they can vote for the measure, which will be a three-line whip. Some feel the £1.5bn saving will have a painful price.
MP for York Central Rachel Maskell, who told Sky News she would abstain, said the swift timing of the vote, and lack of assessment of its impact, has left many concerned - not just those on the left sceptical about Sir Keir's leadership.
A House of Lords committee which scrutinises secondary legislation said it had been introduced without proper evidence of its impact.
A former member of the shadow cabinet, who will be reluctantly voting for the measure, told me he expected the chancellor to be forced to make changes in the run-up to the budget.
In an interview this weekend, Sir Keir stood firm, saying there would be no change in course - as well as further difficult decisions coming down the track.
He will head to Brighton in the morning in a big moment for an incoming Labour prime minister - addressing the Trades Union Congress (TUC) annual conference.
He will be braced for criticism, with major union leaders including Sharon Graham, general secretary of Unite, and head of the TUC, Paul Novak, piling the pressure on and saying he should U-turn.
Sir Keir knows the cut will get through parliament and has shown he can be ruthless, having withdrawn the party whip from MPs who voted to axe the two-child benefit cap.
But Labour MPs who back the measure through gritted teeth, and feel it's had too high a price, will be harder to win over next time.
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