UK Prime Minister Keir Starmer is girding for what could be the most consequential vote of his premiership, with dozens of Labour rebels expecting to vote against the government’s welfare overhaul despite huge concessions to push it through parliament.

Starmer spent Monday trying to persuade skeptics in his left-leaning party to back a revised package of reforms to disability benefits that he says are necessary to slow the pace of Britain’s ballooning welfare costs and get people back to work. Some of the party’s Members of Parliament said they weren’t satisfied by sweeping last-minute changes offered by the prime minister last week at an estimated cost of £3 billion ($4.1 billion) to the Treasury.

Debbie Abrahams, one of the leading Labour MPs who had negotiated the concessions with Downing Street, said she wouldn’t vote for the bill when it returned to House of Commons on Tuesday. “The actual offer that was put to the negotiating team wasn’t what we thought,” she told ITV News. “We implore the government to think again.”

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Another leading rebel told Bloomberg News that, despite initially expecting to vote with the watered-down plans, they weren’t reassured by the details. Still, another prominent Labour MP said they believed the government would carry enough votes to prevail.

As of Tuesday morning, 39 Labour MPs had signed a new amendment aimed at sinking the bill, with government whips fearing the number of rebels could rise as high as 50. While that would represent a significant challenge to Starmer’s authority, it wouldn’t be enough to defeat the government.

Nonetheless, Business Secretary Jonathan Reynolds repeatedly declined to predict the outcome of the vote. “It is decision time,” Reynolds told the BBC. “This is a genuine set of improvements. I feel very strong in putting that case to colleagues today.”

Starmer will hold a meeting of his cabinet on Tuesday morning, before the debate starts in the House of Commons around lunchtime. The vote is expected by 7 pm.

Last week, more than 120 Labour MPs had publicly threatened to oppose the legislation. That was well in excess of the 83 needed to succeed alongside opposition in killing it.

Any defeat on the floor of would deal a seismic blow to a government that was propelled into power with an massive 165-seat majority almost a year ago. A prime minister hasn’t seen a bill fail at a similar stage since 1986 and losing such a showdown would cast new doubts about Chancellor of the Exchequer Rachel Reeves’ fiscal plans.

Defeat would undermine the government’s authority and would bolster those calling for the replacement of Starmer, and his chief of staff and top political adviser, Morgan McSweeney. Starmer acknowledged in an interview with the Sunday Times newspaper that he was too distracted by events abroad, including the NATO summit in The Hague, to focus on the rebellion.

Nevertheless, as of Monday evening, there was little sign that Starmer was preparing to pull the bill. When asked if the government was confident it would win on Tuesday, Starmer’s deputy spokesman, Tom Wells, declined to comment to reporters, saying he didn’t want to get ahead of the vote.

Instead, Work and Pensions Secretary Liz Kendall mounted a vigorous defense on Monday of what remained of the overhaul in the House of Commons, with Reeves looking on.

The chancellor had argued in March that the reforms were necessary because an extra 1,000 people a day were signing on for Personal Independence Payments, a benefit paid to disabled people to help with their living costs. The payments had been projected to almost double to £41 billion by the end of the decade, while overall spending on disability and incapacity benefits is set to rise to £100 billion from £65 billion last year, according to the Office for Budget Responsibility.

Still, Labour lawmakers were cool to the proposals, especially after a similar cut to winter heating subsidies was blamed for pushing voters to Nigel Farage’s populist Reform UK in local elections in May.  

Starmer returned to London ready to make an expensive U-turn, agreeing that existing PIP claimants wouldn’t be subject to the stricter criteria the government had been seeking to introduce. He also committed to consulting on further PIP reforms with disabled people. Even after those changes, an official impact assessment showed the revised measures would still plunge 150,000 more people into poverty, a forecast that was seized upon by the bill’s opponents.

While government officials initially expected that a rebellion would be limited after the changes, concern grew on Monday after precise details of the concessions emerged. Kendall told parliament that the tougher new requirements on some disability payments would be imposed alongside the results of a wider review of the system led by disability minister Stephen Timms.

In a potential warm-up to a parliamentary showdown on Tuesday, several Labour MPs publicly questioned the government’s approach. “What is the logic of making changes to future claimants before producing the Timms review?” Sarah Owen, the Labour chair of the women and equalities select committee, asked Kendall.

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Many Labour MPs said they were still making up their minds ahead of the debate.

Other senior Labour politicians added to the pressure. London Mayor Sadiq Khan said he had “serious concerns” while his counterpart from Greater Manchester, Andy Burnham, urged his colleagues in the House of Commons to “vote against the whole bill.” He said the changes would create “unfairness and a divide” among disabled people, describing the government’s changes as “half a U-turn.”

With such divisions in the governing party, the opposition Conservatives needed to do little than join in. Helen Whately, the Tory’s shadow work and pensions secretary, described the changes as a “rushed and chaotic compromise.” Conservative leader Kemi Badenoch said her party would vote against the legislation.