There are two questions for Tory MPs today.
1) Do they believe that the PM will cost them or save them their seats and livelihoods at the next general election?
2) How pernicious to confidence in the important institutions of government - and therefore to confidence in British values - is the widespread perception that the politician at its apex is dishonest.
The first question is about the future of the Tory Party. The other is about the future of the UK.
They are of course linked.
Tory critics of Mr Johnson say that he is now - in the words of one - “our Jeremy Corbyn”, by which he means that that perception of Johnson’s lack of scruples means large numbers of potential Conservative voters will never again vote for their party while he is leader.
But Mr Johnson’s supporters say he is the most formidable campaigner their party has known in the modern era, and if anyone can turn round the ailing fortunes of the Tories it is him.
So this vote matters to Tory MPs and the whole country.
The widespread expectation is that Johnson will win tonight. But no Tory MP to whom I have spoken believes he can stay PM for more than a few days if he were to win by just a single vote - despite what his licensed spokes people say.
Being supported by just over half of his MPs is no mandate, because it would mean that the vast majority of backbench MPs - who do not owe their ministerial jobs to his good will - would be against him.
But if he secures two thirds of the vote, he’s safe in the job at least for a year - the time limit before another vote can be held - and possibly till at least the next general election.
It is in that no man’s land of Johnson winning somewhere between a half and two thirds of the vote where the Tory party would have a dilemma - to decide whether or not he retains sufficient authority.
And if you hear anyone in the coming hours forecasting the result with apparent certainty, you are listening to a fool or a propagandist.
We are about to witness a secret ballot of a tiny electorate most of whom have a powerful motive to lie about how they plan to vote.
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