It's finally happened and the 48 letters which are needed to trigger leadership challenge have finally been received by the Chairman of the 1922 Committee, Sir Graham Brady.  What the Australians would call a 'leadership spill' will take place tonight, and voting in a vote of confidence in the Prime Minister will take place between 6pm and 8pm. If she loses by one vote she's out. Ousted by her own colleagues. 

The numbers here are important. The fact that it has taken so long to garner the 48 letters could indicate that it will be almost impossible to get the 159 votes needed to oust her. In normal circumstances that number might not be the one which mattered. If the Prime Minister loses the support of 30-40% of her colleagues, that ought to be enough for her to resign and let someone else take the Brexit process forward. But we are not in normal circumstances and given Theresa May's limpet like grip on power, I do not expect her to relinquish it without a fight. But that fight may not be with her MPs it may be with her cabinet. In that famous Newsnight 'Tower of Power' the Cabinet now rises up a few notches. Her future may be in their hands. There may well be quite a few of them who calculate that if she goes, their own Cabinet futures will be uncertain to say the least. Looking down the list of members of the Cabinet I would say at least half of them would not survive any expected leadership change. That will concentrate a few minds.

Judgment Day

There may be further consequences today. There could be more Cabinet resignations. Surely if there is any honour left in politics if a Cabinet minister is to vote against their own Prime Minister they would have to quit. I reckon there are at least four Cabinet ministers who will vote against the Prime Minister. There will be many other junior ministers.

In terms of the timetable, if there is a change of leader, the parliamentary rounds would have to be completed next week. Depending on the number of candidates, there may have to be multiple rounds over two days. Nominations would presumably be closed by the end of tomorrow with hustings taking place on Monday, and the first round(s) of voting taking place on Tuesday. There would be no hustings around the country, but I suspect there would be two sets of TV/Radio hustings during the first week of January, with ballot papers being sent out at the same time, with voting closing on January 14th. That's the quickest it could be done. David Gauke is saying it couldn't be achieved until the end of January. Not true.

Of course the other scenario is that there is a coronation. The voluntary party would be outraged at that prospect (again), but sometimes needs must. I think that largely depends on the result of the final round of voting. If there is a candidate in an unassailable lead, there would be a lot of pressure on the second placed candidate to stand down.

Going back to the numbers, if the number of MPs voting against her is under 100, she may be safe. If 104 vote against her that's one third of the parliamentary party. If 125 vote against her, that's 40%.

I'm not going to make a prediction because it's a fool's errand. This is the most duplicitous electorate you'll ever come across. But for those who think it's a done deal that she's toast, I urge them to remember how long it's taken to get to the point where the 48 letters have been sent in. I know a fair few MPs who've told me they wouldn't send in a letter but would certainly vote for a change of leadership if they had the chance. I doubt very much whether even that will be enough to get to the 50% + 1, but in politics at the moment anything is possible.

In terms of runners and riders if she does go, I think it's a dead cert that the five people certain to stand will be Boris Johnson, David Davis, Dominic Raab, Amber Rudd and Sajid Javid. In an ideal world, the number of candidates would not extend beyond that, but if you remember my article on Saturday (click HERE) I identified 36 - yes, 36 - potential candidates. Most of them will hopefully discover some self-knowledge over the course of the next 24 hours and do the adult thing and not stand. 

Constitutionally, this ballot affects the future of the Leader of the Conservative Party, and not the Prime Minister. Even if she loses this vote tonight, she remains Prime Minister. However, it's entirely possible she could immediately stand down, with David Lidington most likely taking over as a stop-gap prime minister. 

One final thought. The optics of the Conservative Party ousting a second female prime minister are not good. We all remember that tragic scene in November 1990 of Margaret Thatcher leaving Downing Street in tears. That was 28 years ago, yet it seems like yesterday to many of us. 

As a political broadcaster I obviously live for days like this. I'll be on air on LBC this evening while the vote is taking place and when the result is announced. It'll be quite a moment. I hope you'll join me.