After almost two months of political paralysis, 12 hustings, countless policy launches and relentless mud-slinging, the UK’s Conservative leadership contest finally ended at 5pm on Friday. On Monday, Britain will learn the identity of its next prime minister.

Boris Johnson’s successor — either the foreign secretary Liz Truss or former chancellor Rishi Sunak — will emerge from a brutal contest that has left many Tory MPs despairing. “It has been unprecedented both in terms of the poor calibre of the debate and the level of vitriol,” said one former cabinet minister.

The widespread expectation among pollsters and protagonists is that Truss will be victorious, having grown through the contest and shown a better understanding of how to woo the 160,000-odd Tory members who have the final say.

The damage caused by the protracted contest will take some time to repair. Dominic Raab, deputy prime minister and Sunak supporter, has accused Truss of planning an economic policy based on tax cuts that would be an “electoral suicide note”.

Meanwhile Truss’s team rewrote the lexicon of political insults, accusing Sunak of “thrashing around all over the place like a wounded stoat” and of “aggressive mansplaining and shouty private school behaviour”.

Initially there was a feeling Boris had to go, then time lapsed and the membership became less sure

Sunak’s allies admit privately that Truss has run a good campaign, after a very poor start, astutely playing into the Tory grassroots enthusiasm for tax cuts, Thatcher worship and attacks on “woke culture”.

But Sunak’s team believe the biggest single factor behind his likely defeat was his role in toppling Johnson on July 7. The weary political cliché that “he who wields the knife never wears the crown” has been exhumed.

Sunak’s resignation as chancellor in July precipitated Johnson’s downfall; at the time Truss was fortuitously on the other side of the world at a G20 meeting in Indonesia. Sunak’s allies admit: “The nostalgia for Boris came swiftly. Opinion in the party hardened in a very different place.”

A YouGov poll of Conservative members just ahead of Johnson’s resignation found that 35 per cent thought he was wrong to quit; a sizeable number, but perhaps not insuperable. That figure had grown to 51 per cent by the time voting started in early August.

At the first hustings, Sunak was asked by a Tory member if he had “stabbed Boris Johnson in the back”. Nadine Dorries, culture secretary and a Truss supporter, retweeted an image of a toga-clad Sunak plunging a knife into Johnson.

“Initially there was a feeling Boris had to go, then time lapsed and the membership became less sure,” admits one Sunak ally. “That was the biggest single factor in the race.”

Truss’s team agree that Johnson nostalgia was a big factor in the campaign. It could also become a problem for Truss if she becomes prime minister should things go wrong and some in the party start hankering for a return of Johnson before the next election.

But Truss’s supporters also believe that Sunak’s attempts to interrupt and talk over the foreign secretary at the first television debate in Stoke was a “massive, massive” mistake. Sunak’s allies agree.

“It’s hard to overstate just how wrong they got it by sending Rishi in to try and bully and mansplain Liz,” said one senior adviser to the foreign secretary. “For days, all we heard from Tory women aged 49-plus was how he shouted over her.

“Leaving Stoke, honestly, I thought we were home and hosed. If you want to look at what swung the contest it was the debate, taxes and the fact that Rishi knifed Boris,” they said.

Truss was soon urged to refrain from risky new policy moves, not least after her plan to cut £8bn from the public sector pay bill outside London was quickly aborted after a furious response from northern Tory MPs.

Instead she spent the latter part of the campaign with her head down — ducking out of a planned BBC interrogation with the journalist Nick Robinson — and preparing her team and policies for government.

Simon Case, cabinet secretary, and his team held “transition” talks with Sunak’s too, but the focus was on Truss and her new court, which hunkered down at Chevening, the foreign secretary’s Kent retreat.

Truss’s chancellor-in-waiting, current business secretary Kwasi Kwarteng, will be a key player in the new administration. He will deliver an emergency Budget in the coming weeks to confront the incoming economic storm: by far the biggest challenge facing the government.

Sunak’s team reckon Kwarteng will be obliged to spend more than £100bn, both on the tax-cutting promises Truss wants to enact — such as a £30bn plan to reverse corporation tax and national insurance rises — and the things she will have to do.

They include a massive package of support for households — which she described as “handouts” in a Financial Times interview — alongside help for small businesses struggling with spiralling energy bills and for public services.

With interest rates rising and the markets starting to bet against the British economy, the cost of servicing the national debt is sharply rising, putting pressure on Truss to drop some of her more costly plans. Many Tory MPs fear the worst in the coming months.

“Every conversation I have with a colleague leaves me feeling more depressed,” said one former cabinet minister, who fears the fragile Tory electoral coalition of rich southern seats and former Labour working class constituencies in the north could fall apart.

The fear among Tory MPs is that Truss could be both too rightwing for the north — many of whom backed Johnson in the hope of more state spending — and too populist and Brexit-focused for wealthier graduates in the “blue wall” Leave areas of the south.

“A party of the centre-right should be a party for successful people,” sighed one senior Tory MP, noting that party campaign strategists had designated five seats in the Surrey stockbroker belt as being vulnerable at the next election.

But while the leadership contest may have been bruising, the task ahead for Britain’s new premier is even more daunting. “I’ve never known an in-tray like it for an incoming prime minister,” said one veteran Tory MP. “Everything is in it apart from Armageddon.”