Theresa May said "difficult days lie ahead" as she announced her Brexit deal had been approved by the Cabinet.
One of the chief obstacles could be the House of Commons, where a simple majority of MPs will need to vote for the blueprint for the deal to be given the green light.
The magic number is 320, a majority of the 639 voting MPs in the Commons which excludes suspensions, the Speaker, three Deputy Speakers and seven Sinn Fein MPs who abstain from attending the UK Parliament.
The Parliament website lists 93 MPs who are ministers of Government and would therefore be bound to support Theresa May’s Brexit plans owing to collective responsibility.
In addition, loyalists to Mrs May and the Government would vote in favour of the deal, as well as a number of Brexiteer MPs who may have been spooked by the PM’s message that it was this deal, no deal or no Brexit.
If all opposition parties voted against the deal, and they were joined by up to 40 Conservative rebels, the Government would be as many as 44 votes short of the 320 they need.
The Liberal Democrats, Greens and Plaid Cymru have said they are opposed to a deal, and it seems unlikely the Scottish National Party would support it given Nicola Sturgeon said the agreement would be “a bad one for Scotland”.
The Democratic Unionist Party’s stance on the Brexit deal is uncertain. Arlene Foster has said the Prime Minister is "fully aware of our position and concerns” over Northern Ireland being treated differently to the rest of the UK.
There is division within the Conservative party too, with the European Research Group, led by Jacob Rees-Mogg, claiming it has 80 MPs ready to vote against what it sees as Mrs May’s capitulation to Brussels.
Opponents suggest the number would be whittled down under a concerted offensive of arm-bending by the Tory whips, so it may finally be considerably less.
This means Mrs May could need to find up to 100 votes - but perhaps more realistically somewhat less - from elsewhere.
So the Prime Minister may well need to extend an olive branch to Labour, which also has its own divisions between those like Chuka Umunna who oppose Brexit, those like Caroline Flint who support it reluctantly because their constituents voted to leave, or those like Kate Hoey who are ardent Brexiteers.
If the Government was able to persuade as many as 40 Labour MPs to back the deal, and limit the number of Tory rebels to around 20, this could be enough to reach the magic number of 320.
The question is how many of Jeremy Corbyn’s MPs are prepared to defy him and support the Prime Minister, reasoning that a no-deal Brexit, which they fear could bring about an economic disaster, would be a worse outcome.