The shock resignation of the cabinet member Nicky Morgan last night speaks volumes about the nature of this Conservative government – present or prospective – that she no longer wishes to be a part of. The rightward march of a Boris Johnson-led party is something a surprising number of Tory MPs have decided they can no longer stomach, as they jump ship with unconvincing mumblings about spending more time with their families.

Standing down suddenly, not just from the cabinet but from her seat and from politics altogether at age 46 in the prime of her career, is a drastic act that has taken her colleagues by complete surprise. Politics is not a job, but a way of life: a mode of thinking and being, a social world, a friendship group, almost a cult. It’s an addictive activity that can take years to get over in the rehab of normal life. The yearnings of MPs ejected or retired can last a lifetime – especially those not lucky enough to slip into the rest and recovery zone of the House of Lords.

Morgan cited the abuse politicians – particularly women – receive as one of the reasons for her departure. But in leaving ahead of an election , she is also something of a bellwether for the Tories, given that she is essentially a moderate who has tried to navigate the rollercoaster of the great Brexit and Johnson schism in her party.

She was a remainer, but a compromiser as she searched for the unicorns of “alternative Brexit arrangements”. Finally she seemed to succumb to Brextremism when she accepted a cabinet post and took the Johnson loyalty pledge, to the distaste of some erstwhile allies. What’s more, she seemed set to promote a Johnson wrecking agenda when as culture secretary she toyed with the possibility of abolishing the BBC licence fee for a Netflix-style subscription service. Along with the NHS, the BBC is a Tory totem of loathing for national treasures beloved by the people. The very success and popularity of these institutions is an affront to the beliefs of ardent free-marketeers. But plainly, something snapped. She is off, before her hands are sullied by what’s to come if her party wins.

The disgraceful treatment of Amber Rudd stunned many, says Dominic Grieve, the whipless ex-attorney general, himself setting off on the lonely path of running as an independent in his 65%-Tory seat. “It was vindictive and vengeful after she asked for the whip back. Of course it came straight from Boris himself,” he says.

The rollcall of departing moderates amounts to a cull of Ken Clarke-admiring Tories, many of whom attended the last One Nation dining club on Monday night; Morgan, who didn’t reveal her plans, was among those. As parliament dissolves, that largely remainer dinner may be looked back on as the last gasp of a dying epoch. Away go David Lidington, Alan Duncan, Justine Greening, Oliver Letwin, Alistair Burt, Philip Hammond, Rory Stewart, Seema Kennedy and a score more, along with the prime minister’s own escaping brother, Jo Johnson – following those who have already fled the party: Heidi Allen, Anna Soubry, Sam Gyimah and more. “We are seeing the complete collapse of moderate Conservatism,” says Grieve, in sorrow and alarm. The Times estimates that of 43 members of the One Nation group, only 26 are standing again for the Tories – it adds that Steve Baker, chairman of the hard-right European Research Group, expects its numbers to swell after the election.

Outsiders may fail to see much change in the party that has headed for Hayekian free marketeering, privatising and bonfires of red tape ever since Margaret Thatcher’s day. The party that chose Iain Duncan Smith and Michael Howard as leaders, followed by David Cameron, whose deceptively modern facade hid a small-state ideology that shrank back public services beyond anything Thatcher dared. But it would be a mistake to dismiss the warnings of these Tory moderates packing their bags, disgusted by what they fear is to come next.

The term one nation has been rendered virtually meaningless from overuse by every Tory of every hue, to well-deserved mockery from opposition parties. One of the One Nation group’s founders was Enoch Powell, when they saw it as signifying a modernising willingness to accept the 1945 settlement and embrace the welfare state instead of fighting it.

Now Boris Johnson claims it for himself: “We’re dealing with a chameleon,” says Grieve. “He adopts whatever persona will please.” What do Grieve and the dining club mean by the term? “I’ve always understood it to mean quiet, moderate government,” he says. And he expects anything but that from a Johnson/Dominic Cummings regime.

What would signal that rightward move? “Deregulation will be the key issue, and an aggressive neoliberal economic policy,” Grieve says. At the dinner, he found many were “really gloomy” as they mulled over a Johnson victory. It would mean departure from the EU on 31 January and an abysmal negotiation of the future EU relationship, where refusal to accept European regulatory standards would be the blocking point.

The sea change will come not just through the lost moderates, but the selection of new Tory MPs by the same old Europe-hating, backward-looking local parties that chose Johnson as their leader. Professor Tim Bale, analyst of political parties, points out that since the late 1990s it has been very difficult for anyone but firm Eurosceptics to be selected in any seat.

The party, he says, has a history of oscillating. On social issues such LGBT and women’s rights, it’s unlikely to retreat from what he calls the “silent revolution” of social liberalism. But Johnson is taking hard lines on punishment, law and order and immigration. Hanging in the balance is environmental concern: the odds are, he will follow Cameron and “drop the green crap”, satisfied with kindness to animals and abolishing plastic straws.

If socially liberal issues of personal freedom are safe, there is, says Bale, the “silent counter-revolution” moving the other way, embracing nationalism, distrust of others and an anti-immigration, hostile environment.

Heading into the election, Johnson is already turning the Tories into an ersatz Brexit party. That is the party from which these Amber Rudd, Nicky Morgan, Alistair Burt escapees are fleeing.

• Polly Toynbee is a Guardian columnist