Theresa May’s biggest and most important challenge tomorrow in giving her vision of the UK’s future trading relationship with the EU is to persuade the rest of the EU that her ambitions are consistent with EU lore and law and would be approved by the UK’s parliament.
It will be horrifically difficult for her - the more so since she since she has the combined weight of the Labour Party, Conservative rebel MPs and the former Tory prime minister Sir John Major telling EU leaders that most parliamentarians would prefer a much more conventional settlement with the EU than what the PM will propose.
Theresa May wants a “deep, special and bespoke” deal that looks a bit like a customs union for goods, and like single-market membership for financial services, but would not impose on the UK - as arbitrator of whether we are following the new rules - any role for the European Court of Justice, or any prohibition on our ability to negotiate our own trade deals with the likes of the US and China.
Her red lines - no role for the ECJ, freedom to negotiate third party trade deals, no more free movement to the UK of migrants from the rest of the EU, no more substantial payments into the EU’s budget - are also the powerful and substantial reasons why the rest of the EU cannot and will not agree a trading relationship with the UK that is anything but substantially worse for us than our current commercial relationship.
But to be clear, they are not going to formally agree a deal with the UK for years. What is at stake is whether, at a summit later this month, EU leaders decide May has said enough that is practical and realistic to allow negotiations to commence at all.
And that judgment will be determined not only by what she says but also - as I mentioned - by what they are hearing from the likes of John Major, Jeremy Corbyn and pro-EU Tories such as Anna Soubry and NickyMorgan.
The point is that Merkel, Macron and the rest will be much less likely to bend in May’s direction if they fear she could not get the deal she wants from them ratified in her own parliament.
That is a legitimate fear. The mood of most MPs, though not most Tory MPs or the cabinet, is for the UK to stay in “a” or “the” customs union - to protect manufacturing jobs and to keep the border between Northern Ireland and the Republic as open and permeable as possible (and therefore in theory preserve the peace).
So why should the rest of the EU compromise with May on a trading relationship different from that, different from what the real seat of British sovereignty, parliament, may want?
To put it another way, May's number one priority tomorrow is to prove that what she seeks from the EU as a future trading relationship isn’t simply some unworkable fudge cooked up by a dysfunctional cabinet - but is the basis of a deal that parliament (and perhaps at some point the people too) would ratify.
For what it is worth, the prime minister believes the permanent Brexit war of egos and ideas in her cabinet is a virtue - because it means the policy that emerges captures the myriad views of the British people themselves.
It is not just the ending of pernicious uncertainty about what Brexit means that hinges on her being right. So she would not survive parliament voting for a Brexit other than hers.