LIKE any good prosecutor, Sir Keir Starmer knows how to twist the knife. 

“Does the Prime Minister think that he has the moral authority to lead and to ask the British people to stick to the rules?” he asked Boris Johnson at PMQs.

The question, about Downing Street partying blithely through last year’s Covid lockdown, naturally drew derisive snorts. 

It says something about the Prime Minister that putting him in a sentence next to “moral authority” is automatically considered sarcastic, but there we are.

It was a neat way to illustrate the debased state of the Government. 

Sir Keir finessed this cold-blooded shanking by contrasting the country’s elected leader with its head of state.

Downing Street party: It's official – everyone had a great time at the No10 party that never happened

“We will face other tests where the British people may be asked by their leaders to make further sacrifices for the greater good,” said the Labour leader. 

“Her Majesty the Queen sat alone when she marked the passing of the man whom she had been married to for 73 years. 

“Leadership, sacrifice - that is what gives leaders the moral authority to lead.”

Obviously BoJo is no Lizzo, but he is becoming increasingly alone.

Not only did he lose his adviser Allegra Stratton over the mock press conference in which she joked about one lawless party, he is also losing his backbenches.

In times past, Tory MPs could have focused on the next holiday and hoped to draw a line under matters and start off on a fresh foot when they returned.

‘Let’s just get past Christmas,’ they might once have consoled each other. 

But by now it’s dawning on them that there is no respite ahead. No safe harbour ahoy.

There will be no let up from the sins and failings of Boris Johnson as long as he’s in Downing Street, because that is who he is. 

They backed him knowing his slapdash, conceited ways, but thought his sunnier side would prevail with the electorate.

Not any more. A grin and a wink and a rustle of the old thatch no longer cut it. 

Now the scandals come hard on each other’s heels like winter storms.

They may not originate with the PM, but he has a gift for compounding them. 

The manifest inadequacy of his response to the party row, for instance. 

It is inconceivable he knew nothing of the event until the leaked video of Ms Stratton at full giggle, yet he batted away concerns and insisted all was well.

He might have got away with that kind of patronising twaddle on a different day on a different subject, but not after two years in post on a life-or-death issue which has directly affected all of our lives.

It is also inconceivable that the brazen mendacity that characterised Number 10’s response would have happened without Mr Johnson setting the tone with his own waywardness with the truth.

Tellingly, Mr Johnson told PMQs he was furious “to see that clip”, rather than furious to learn what had been going on under his roof.

And while he apologised, he also failed to accept that, very clearly, there had been a party, and that such a party would have breached lockdown rules at the time.

Instead, there was whining about Labour “playing politics”, underlings thrust under buses, and a Cabinet Secretary inquiry that will look only at the December 18 shindig, despite allegations of others, including one in his own flat.

Tomorrow, the PM’s disaffected former aide Dominic Cummings - who tweeted hints of yet more Downing Street parties yesterday alongside calls for ‘regime change’ - will hold an online Q&A on his Substack blog.

“Ask me anything,” he says ominously for his old boss.

SNP Westminster leader Ian Blackford said Mr Johnson should quit. There’s no sign of that yet, especially as Ms Stratton’s departure might draw the sting a little.

But the PM risks gravely overestimating his popularity and longevity. 

He has a reputation as a winner on the basis of the 80-seat majority he pulled off in 2019, even though large numbers of voters backed the Tories out of weariness with Brexit and wariness of Jeremy Corbyn, rather than love of Mr Johnson.

The PM’s allies also remind us he won the London mayoralty twice. He did. Both times against Labour’s Ken Livingstone, who hadn’t read his own sell-by date.

In 2008, Mr Livingstone had already been doing the job for eight years and tried to go for the treble. When the final transfer votes were counted, he managed 46.8 per cent of the vote to Mr Johnson’s 53.2%.

Four years later, Mr Johnson’s vote dipped to 51.5%, while Mr Livingstone’s rose to 48.5%. Mr Johnson tally of 1m votes was 100,000 down on 2008.

Mr Johnson entered the Commons as MP for Uxbridge and Ruislip in 2015. 

He had inherited a safe Tory majority of 11,216. By 2017 it had halved to 5,034. 

Even after he became Prime Minister, he only nudged it up to 7,210 in 2019.

It’s as if the more voters see of him up close, the less they warm to him. The same may well now be happening nationwide.

Next week, Tory eyes will be on North Shropshire and the byelection caused by the resignation of MP Owen Paterson over a parliamentary sleaze scandal.

Mr Johnson tried to protect his pal by changing the standards regime that caught him, then had to back down, triggering a wave of self-inflicted sleaze stories. 

Road-testing an electoral pact to dislodge the Tories at the next election, Labour have let the Liberal Democrats take the lead, with sleaze a key factor.

If the Tories lose, or their 22,949 majority is shredded as voters turn against the Prime Minister, then it won’t just be the opposition urging him to make tracks.

We had an early taste of that yesterday as the current and past leaders of the Scottish Tories ran a mile from him.

Douglas Ross said Mr Johnson would have to go if he had misled parliament. 

While Baroness Davidson, despite being ennobled by him, said his lines at PMQs had been “pathetic”. 

In these serious times, Mr Johnson already cuts a profoundly trivial figure.

If he also looks like an electoral loser to his party, his time will run out fast.

Not all political careers end in failure. Some also end in dishonour and disgust.