But they have won a temporary reprieve, because Sue Gray - the senior civil servant investigating the alleged rule-breaking parties - will delay publication of her report until the Met Police has conducted its own investigation of whether the Covid laws were breached and whether fixed penalty fines should be levied.
The big question is whether the decision of Met Commissioner Cressida Dick, to investigate, and the associated stay of judgment for Prime Minister Johnson, is good or bad for him.
My judgement, and those of senior Tories, is that it is net bad.
Because it is straightforwardly a terrible look for a prime minister to be at the centre of a police investigation. Some will say it gives the PM more time to prepare his excuses for whenever the Met reaches its conclusions and Gray publishes, and gives him more time to lobby his anxious MPs that this is not the time to throw him out.
But the Gray report will be what it is, whenever it is published. And those close to the investigation tell me that although she will not pass explicit judgement on whether the PM has lied to public and Parliament, breaching the ministerial code - and will not do so because that is not her remit - neutral readers of what she eventually publishes would struggle to reach any other conclusion.
Apart from anything else, the Met would not be investigating if there were not prima facie evidence, established by Gray, that the rules were broken.
This is not lost on Tory MPs. It is causing them grave concern.
So whenever her report is published, it will be one of the most challenging moments in Johnson's career.
As for officials in Downing Street, their stress levels at being investigated by police as well as by one of their own must be off the charts.
Good government is almost impossibly difficult in these chaotic circumstances. What a mess!
The Met has not yet told Gray quite which putative parties they are probing, though we know they are not looking at all of them.
It is therefore hard to assess how long this extended detective process will take.
In the meantime, the prime minister's best hope is that public anger will fade as the weeks pass, and other issues come to the fore.
Cressida Dick confirms an investigation has been launched:
That may be a slightly naive hope, with local elections looming in May, and the Labour Party massively incentivised to keep partygate alive day in, day out.
The greatest concern for Tory MPs is that in granting a reprieve of weeks or months for the PM, they make it impossible for them to escape the reputational horror of partygate.
There will be no opportunity for the Tories to relaunch with a clear slate, no compelling means to rebuild their popularity, till Met and Gray conclude their work.
Out of a sense of natural justice, Tory MPs' instinct will be to delay their own judgement on Johnson till they have heard from the police and Sue Gray.
But in being fair to the PM, their party will remain in the dark shadow of partygate and may become weakened and damaged beyond immediate repair.