At just before midday today, Brexiters will own Brexit for the first time, and that will really matter – if, as expected, Johnson is crowned Tory leader.
Because from that moment, they will have no one but themselves to either praise or blame, for either Britain's brave new dawn or its slow and painful demise (whichever turns out to be our fate).
The point is that, till today, Brexiters inside and outside the Tory party, from Jacob Rees-Mogg to Nigel Farage, have been able to attribute the failure to deliver Brexit, and an absence of the great sense of national rebirth that they expected, on the treacherous and pesky Remainers, who currently occupy all four of the great offices of state.
If it hadn't been for Theresa May as PM, their argument runs, the UK would already have left the EU and would be basking in the glorious sunshine of independence.
Well maybe. Or maybe not.
The important point is that at 11.48 today, the politician who more than any other sold Brexit to the British people, Boris Johnson, will be in charge.
He created Brexit. And now he will own it.
Which you should probably cheer about, whether you love or hate Brexit. Because there is no moving on from the terrible political paralysis undermining the business of government and confidence in government till Brexiters properly own what they made.
Which is not to aver that Brexit will end up a great success – or per contra that it will be a catastrophe and we'll beg the EU's leaders to let us reverse our decision to leave.
But it is to say that we can't know whether Brexit is tonic or poison till those who really and truly believe in the Brexit mission are firmly and conspicuously leading that mission.
For the avoidance of doubt, Johnson gets this. It's why, in his leadership campaign. he had only one policy of note or substance: which is that the UK MUST leave the EU on the due date of 31 October, with or without negotiated terms of exit.
Having offered 160,000 Tory members almost nothing else, Johnson also desperately needs them to give him a clear mandate, with an overwhelming majority of at least two thirds of votes cast.
Because that will help his argument that Tory MPs such as the soon-to-quit ministers Hammond, Stewart and Gauke, or the senior backbenchers Greening and Grieve, will be betraying the party that succoured and nurtured them if they carry out their threats to destroy his government rather than collaborate in a no-deal Brexit.
So for Johnson the margin of victory today really matters.
That said he has been trying to bring his arch critics, the Gaukward Squad and beyond, into his tent by requesting and holding meetings over the past two days with Gauke, Rory Stewart, Greening, Greg Clark, Hammond, Grieve et al.
They have not succumbed to his charms – or at least not yet.
One of them said of Johnson and the conversation with him: 'Captain Clueless; they sold a fantasy and now it becomes their nightmare.'
Another: 'Friendly; not entirely reassuring.'
A third: 'He seems keen on do or die by 31 October!'
A fourth: 'My views are well known and consistent, and I'm not going to amend them.'
So Johnson knows there's no unifying his party ahead of Brexit. Which is why the inescapable logic of his position is that the time for Brexit compromise is long past.
It will be a bumpy ride for him, and the whole nation. But as I said on the News at Ten last night, there won't be a no-confidence vote before MPs break for their holiday at the end of the week.
September and October, by contrast, will see parliament in tumult like nothing anyone alive may have witnessed.
And as I think you all know, my central scenario is there will be an autumn general election, fought between a no-deal-Brexit Tory party and a disunited set of opposition parties which have varying degrees of commitment to remaining in the EU (Lib Dems unambiguously remain, Labour conspicuously unsure of its position).
One Tory MP said to me: 'Boris could be the first sub 100 day PM'. Another: 'The hope is a 100 seat majority.'
So the great Brexit adventure begins, after many false starts. Whatever happens to us – and anyone who says they know is an idiot or deluded – it will be momentous: no squibs left of the damp variety.
Robert Peston is ITV's Political Editor. This article originally appeared on his ITV news blog.