Justine Greening has had enough. The staunchly pro-European former cabinet minister, comprehensive schoolgirl and passionate advocate of social mobility has contemplated a future inside a Conservative party that no longer seems to have room for people like her, and chosen to walk away. Threatened with the loss of the whip for voting to take control of the Commons order paper to block Boris Johnson pursuing a no-deal Brexit, she announced that she wouldn’t stand again for Putney. For good measure, she hinted that if a no-dealer is chosen to contest the south-west London seat instead then they’re likely to lose; any successor, she explained, would have to represent her constituents, and most of them voted to remain. Her clear message was that even if Downing Street wins this battle, it could easily lose the war.

They’ll know that if all the moderates melt away then Nigel Farage will essentially have won

Putney is just one of a swath of marginal seats that the Tories can probably hold only by appealing to socially liberal, moderate Tories like her. They may be an increasingly embattled minority, but one in five Tory voters think no-deal Brexit would be a bad outcome for Britain. What are those voters supposed to do when the MPs they naturally warm to start fleeing a sinking ship? For by throwing down the gauntlet to their prime minister, the rebels are also forcing Tory activists and voters, who share their one-nation values of tolerance, political plurality, and a politics of compromise and consensus, as well as fearing the impact of a no-deal Brexit, to confront the choice some have dreaded making. Do they stay and fight for the return of what they regard as sanity to their party, even if it means years of banging their heads against a particularly unforgiving wall, or give up and move on?

Not all will choose the same path. In an extraordinary interview shortly after Greening spoke up, Philip Hammond made clear that he intends to stay and fight, digging in against a deselection threat he doesn’t think Boris Johnson has the power to enforce (he has already been reselected by his local association). As a Tory for 45 years, Hammond said he was going nowhere: “This is my party, I will protect it from those trying to turn it into a narrow faction.”

And if his talk of fighting back against the “incomers and entryists” threatening to hijack his party sounded like the sort of thing hostile Labour MPs used to say about Corbynites, that’s no coincidence.

The speed with which husky-hugging Cameroon modernisers have gone from the dominant mainstream force inside the party to a marginalised one is breathtaking, but hardly unprecedented in recent years. Tory rebels see in Dominic Cummings – who has bragged in the past about not actually being a member of the Conservative party despite working for several of its leading lights – a mirror image of the ex-Communists such as Andrew Murray, who now have Corbyn’s ear. They also see the swaths of ex-Ukippers rejoining constituency associations as cuckoos in the nest, just waiting for their chance to oust sitting MPs who don’t share their views. And now the showdown some have been grimly expecting ever since the referendum is here.

Some of the refuseniks are old school, one-nation Conservatives who are either already retiring, such as Ken Clarke, or have been around for long enough not to care about their future ministerial prospects. Others such as Greening, Nick Boles or Ed Vaizey rose to prominence as part of David Cameron’s efforts to detoxify the Tory brand, and are still young enough to have a second career outside politics if they go now. For every Tory openly voting against the government tonight, there will be even more like-minded MPs who share their fears for the party’s future but are siding still with Boris Johnson, either in the hope he somehow pulls off a deal or under the illusion they can influence things from the inside.

But if there is to be a general election in which Tory candidates might be expected to embrace not only a no-deal Brexit, but potentially a slew of provocative “culture war” policies – Downing Street is said to have polling ideas on trans rights, for example – to woo socially conservative voters back from the Brexit party, then the Amber Rudds, Nicky Morgans and Damian Greens will also have to ask themselves whether they can in all conscience be part of it. And that question will cascade right the way down the party, to councillors and activists and voters wondering if this is still the Conservative party they joined, or whether it has been taken over by fundamentally unconservative forces.

On the one hand, they’ll know that if all the moderates melt away then Nigel Farage will essentially have won; the transformation of the Conservative party into something virtually indistinguishable from his Brexit party would be complete. They will have seen, too, in the collapse of Change UK how difficult it is to get a new breakaway party off the ground and how hard it is even for household names to get a hearing once they no longer have a platform.

But they have also seen how many Labour MPs’ pledges to “stay and fight” Corbynism translated either into a great deal of staying and not much discernible fighting, or into a vicious and exhausting war of attrition with the membership. If many centrist voters dread the choice awaiting them at a coming election, imagine how centrist MPs feel about it. But they may not have long to make up their minds.

• Gaby Hinsliff is a Guardian columnist