Nor is it whether all or indeed any of the parties were actually organised by him.
No. What will determine his survival is whether he has the faintest chance of persuading his MPs that he can reform the toxic party culture, rather than being part of it.
On the illegality of the assorted parties, there is a loophole - though it is unclear whether it was being exploited when the parties were happening or only as a defence after the event.
The point is that official guidelines for the conduct of essential businesses that employ key workers who cannot work from home didn't explicitly say "no parties".
That omission stems from the presumption of those who drafted the business guidelines that no boss would think parties were allowable, given that the more general national rules - which everyone knew - did say "no parties".
But it is an omission that any clever lawyer would exploit.
Now - for the avoidance of doubt - I have spoken to those running our biggest businesses and institutions and they all say they didn't need to be told "no parties". They took it as read. And they tell me it is "outrageous", "shocking", "unacceptable" that there were parties in Downing Street and the Cabinet Office.
"The prime minister and Downing Street has brought the UK into disrepute," the chairman of one our biggest companies told me. "We are an international laughing stock," said another. "How can we possibly sell the idea of 'global Britain' after this?" said a third.
So the omission in the guidance for essential businesses is relevant to almost no one except the prime minister, for whom it represents partial salvation, in making it more difficult to bring a criminal prosecution against anyone deemed responsible for or attending the government parties.
That is why the prime minister, who joined in the 20 May 2020 party, and those ministers who are defending him insist he thought it was a "work event", despite the incongruity of booze, sausage rolls, his wife Carrie Johnson and political advisers who weren't employed in Downing Street.
Are the prime minister and senior Downing Street officials off the hook?
The relevant test is not legality. It's probity.
ITV News Deputy Political Editor Anushka Asthana explains what is the so-called 'Operation Save Big Dog' plan - denied by Downing Street - to help Boris Johnson keep his job
And as I understand it, the senior civil servant investigating the parties, Sue Gray, shares the general perception that the 20 May party and others were just wrong, counter to civil service best practice and have brought shame on Whitehall and the government.
I am reliably told she will make that clear, in the summary of her report which will definitely be published, and in the full report, which may or may not be published.
"It's not looking good [for the prime minister]," said one of her colleagues.
As bad for the prime minister is the despondency of many of his backbench MP colleagues - who have the power to terminate his role as leader of their party and therefore end his term as PM.
Many of them have been unhappy with him for a while, not least because of how the take home pay of those on modest incomes has been squeezed by benefit cuts and tax rises. But what is causing them real distress is voters' fury about the parties, manifested in hundreds of emails that some of them have received and in official party focus groups.
"The focus groups are the worst I've ever seen," said a senior official.
The decisive moment for many Tory MPs was when the PM said on Wednesday that only now could he see that as soon as he joined the drinking and merrymaking in the Downing Street garden after six on 20 May 2020 that he should have ordered his staff to cease their partying and return inside - and he was sorry he didn't.
At that moment, for many of them, any hope he may have of shifting the blame elsewhere and characterising himself as the disinfector evaporated. Or so a number of his MPs told me.
"How can he now present himself as cleaning up Downing Street when he joined in the party rather than shutting it down?" said one.
In fact the almost hourly revelations of Covid rule-breaking doesn't - as the PM might hope - turn him into the victim of an inherited culture. It shows him as someone incapable of recognising a corrupted institution atop which he swaggered.
One small symbol of how respect for him has evaporated is that next week the disaffected former minister, Caroline Nokes, is printing invitations to her 50th birthday party, with the rubric "Come to a 'work event'".
"He won't be leading us into the next election," said another long serving Tory MP and former minister.
MPs disagree about the timing of his exit. But with the exception of those on the payroll, his ministers, it is almost impossible to find any MP who sees for him a path to redemption.