It will take over 180 MPs to topple Boris Johnson and while more than 54 have sent in letters of no-confidence to trigger a vote many others have remained silent.
All the senior Cabinet Ministers have rushed to the ramparts to defend the Prime Minister, although declarations of loyalty on Twitter do not necessarily translate into votes of support in a secret ballot.
Potential rival Jeremy Hunt has declared his will be a vote for change and Penny Mourdant, the Trade Minister with ambitions, has remained silent.
She announced she is marking 78 years since the Normandy landings in World War II, although everyone knows this is D-Day for Boris Johnson.
Here's all you need to know about the process:
What happens if Boris Johnson wins?
If he wins the vote at 8pm, having addressed backbenchers in person at 4pm, the Prime Minister can carry on as before, in theory at least.
Under the current party rules the leader who survives a no-confidence vote cannot be challenged again for another year which would take Johnson, after four years as PM, into the territory of a general election.
However, rules can be changed. Even if he wins the vote the rules of the party could be changed by a simple vote of MPs and he could face another battle to survive.
How long can he go on for?
The truth is that a leadership challenge is wounding to an already limping Prime Minister, and the number of MPs that vote against him will be shackles which Johnson cannot escape.
Theresa May won her vote of confidence in 2018 and managed another four months before deciding to call it quits.
There are two crucial by-elections, in Wakefield, expected to go back to Labour and Tiverton, where a big Tory majority could crumble, meaning more danger for Johnson.
But Downing Street has already factored in these losses and Johnson has made it clear he will be dragged out of Downing Street by his fingernails.
So the only way to get rid of him would be defeat in a vote of no confidence or defeat in a general election.
What if Boris Johnson loses tonight?
If it is a shocker the Tory party would be without a leader but the country would not be without a Prime Minister.
Johnson would be expected to serve as caretaker Prime Minister, although he might choose to let deputy PM Dominic Raab carry on while the Conservative Party selects a new leader.
How do the Conservatives choose a new leader?
Nominations would open shortly after the vote of confidence being lost and candidates would be whittled down in a round of votes by Tory MPs until two remained.
The two surviving candidates then embark on an election campaign in front of the party membership with hustings up and down the country, probably over the summer, to ensure a new leader is in place for the autumn conference and the beginning of the next political term.
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