It is not currently looking hopeful that Boris Johnson's Brexit deal will pass in the Commons. The Prime Minister will need to convince a good number of Labour MPs and independents in order to get over his lack of a majority and the DUP's current refusal to support the government. There are also a number of internal Tory dynamics at play here.
The European Research Group has not yet announced its official position on Boris Johnson's Brexit deal, but already there is a strong chance it could diverge from the DUP. The Brexiteer group was split at the third vote on Theresa May's deal, with dozens of its members voting with the government rather than following the DUP into the 'No' lobbies.
There is an anxiety amongst some ERG members that if they don't vote for this deal, they will become part of the 'Parliament blocking Brexit' narrative which will likely dominate the general election that swiftly follows another defeat. Some Brexiteers also suspect that the DUP will end up coming on board by Saturday anyway - though they admit this is based on a hunch rather than any solid evidence or private conversations.
Of course, the ERG isn't the home of all Brexiteers in the Tory party, and neither does it force members to vote with its leaders. Some MPs, particularly those who are members of the Bruges Group, are waiting for verdict of individual senior Eurosceptic MPs, such as John Redwood and Bill Cash, before they make their decision. Peter Bone, not a member of the ERG but someone heavily involved in the Brexiteer movement, has said he's waiting until he has read the text of the deal in detail.
At the other end of the party spectrum, there are the 21 ex-Tory MPs, some of whom may well use this vote as an opener to returning to the party. Johnson has dropped hints that this will be possible, and while some never want to go back, others are keen to see the whip restored. They are also motivated by the fear of a no-deal exit, and also see this vote as something that could cast them as recalcitrant democracy-blockers, rather than MPs who have been trying to do the right thing. But then there is a further split within this group between those who want a second referendum, and those who think that this too would be calamitous for public trust in politicians. There aren't currently enough of the former for the People's Vote campaign to think it is sensible to press ahead with a call for another referendum on Saturday.
Of course, one of the variables, alongside the shifting sands within each of these groups, is that MPs don't yet have the details of the deal or of how precisely Saturday will pan out. This afternoon, the Commons voted in favour of being able to amend the government's motion by 287 votes to 275. Some MPs have taken this as a further sign that Johnson's deal cannot pass, but as Oliver Letwin, who tabled the move today, said, some who voted against the government today may well on be on board when it comes down to the question of whether to back the deal itself. The government hasn't yet published its motion, and Jacob Rees-Mogg today said it would be 'a motion to either approve a deal or to approve a no-deal exit'.
But a further complication is whether there really will be an extension to Article 50. Jean-Claude Juncker may have said a 'prolongation' is necessary, but he doesn't have the power to rule it out: that belongs to the EU Council. Downing Street is hoping that this will pass many MPs by, and that Juncker's words will make Saturday's vote at least appear to be a straightforward choice between a deal or no deal. But this, as with everything else, is a gamble.