“This is the danger of selling perpetual sunlight and then leaving it to others to explain the arrival of moonlight,” so said Tory MP Huw Merriman last week – letting rip in parliament about the government’s rail plan.

A few Tory MPs referred to those words when I asked about their intentions in a key vote that will take place later today on another complicated and expensive issue – social care.

The accusation, just as with last week’s downgrading of the plans for HS2 and Northern Powerhouse Rail, is that the prime minister – who famously vowed to “end the injustice” of people selling their homes to pay for care – has over-promised. The MPs fear disappointment amongst voters will be inevitable when reality hits.

One former cabinet minister said there was “a lot of discontent” about the issue, especially in the so-called red wall seats in the North and Midlands.

Another senior Tory said that after the last couple of weeks of difficulties for the government on sleaze, “if it feels wrong, why vote for it if they might U-turn and we’ll be left with egg on our faces?”.

Boris Johnson promised, when elected, to “fix” social care. The manifesto on which every Conservative stood vowed that “nobody needing care should be forced to sell their home”.

The policy the government alighted on in September was a lifetime cap of £86,000 on personal care in England for everyone, with means-tested support for those with assets of between £20,000 and £100,000. People with less than assets of less than £20,000 would not be expected to pay towards their care.

It was slipped out last Wednesday that those eligible for state support won’t be able to count that money towards the cap – only their own funds – so stand to lose more of their assets.

As Labour’s Jon Ashworth put it on Sky News this morning: “If you live in a £1million house in the home counties, 90% of your assets will be protected. If you live in an £80,000 terraced house in Hartlepool, or Mansfield, or Wigan for example, you lose nearly everything. That is not fair, that is not levelling up, it’s daylight robbery.”

Were people misled? Junior minister Paul Scully – sent out to defend the policy this morning – conceded that the prime minister’s promise on selling homes had been “boiling down a complicated message”.

As to the criticism from Sir Andrew Dilnot, the architect of the cap, that people on lower incomes would be worse off, Scully said it would be far more generous than the current scheme.

The issue with today’s vote is both the policy itself and what MPs see as the attempt to rush it through parliament.

Tories say while they have been told this change will save £900million a year it’s not clear who is paying for that – how many people on low incomes and where they are located?

The treasury select committee wrote to the chancellor last week to seek clarity on these points and has not yet received a reply.

Ministers sent out to advocate for the policy were not able to answer these questions from MPs. The social care cap has been problematic from the start, but this change has sharpened the concern.

One long-term critic of that policy, former cabinet minister Damian Green, told me: “The flat rate means that you are much more likely to have to sell your house if you live in an area where house prices are lower, which is where most people are not well off, and that is monstrously unfair”.

The social care policy may change further in the coming months, and there are few easy answers to funding it. The government may well win today’s vote, with many Tory MPs keeping their concerns private, and planning to abstain rather than vote against.

But the concern is that when National Insurance rises come in next April, to fund the NHS and social care, those who are not benefitting from it may question what they are paying for.