In order to turn things around with her Brexit deal, Theresa May needs a domino effect. She needs to somehow get 116 MPs to change their vote from last time and back her deal. If the Prime Minister is to have any chance of passing her deal – or significantly reducing the scale of the defeat from 230 votes – May must first convince the DUP that the legally-binding concessions she has secured from Brussels are enough to stop the backstop from becoming permanent. The Attorney General's legal advice could prove pivotal in the matter (and there are doubts in government that he will change it) – though given that Nigel Dodds is a respected legal mind, the DUP's Westminster leader's verdict will be just as important.
If the DUP were to get behind the changes, that would unlock more votes from Tory eurosceptics. The Brexiteers are seen to be in two groups. There are the more hardline members of the European Research Group who have assembled their own legal minds to analyse it and then there are those MPs who took No. 10 by surprise when the voted against the deal the first time. Seemingly loyal MPs like Will Quince and Sir Graham Brady are expected to move if the DUP does. The idea that the Tories would lose the support of their confidence and supply partner if they back the Brexit deal in its original form was enough to put many usually loyal MPs off.
The second group of Brexiteers – figures like Iain Duncan Smith and Jacob Rees-Mogg – will prove harder to win over. As well as DUP backing, they will want their own Brexiteer legal verdict – and here what figures like Boris Johnson and Dominic Raab do could be crucial. If senior Brexiteers get behind the compromises, more ERG members will follow. No. 10 believes there are 20-25 Leave MPs – figures including Anne Marie Morris – who will never vote for this deal.
This is why for the domino effect to work, May needs the DUP and a significant chunk of Brexiteers to get behind the changes. Were the numbers to begin to look close, around 30 Labour MPs could either back the deal or abstain from the vote. There's a view among some Labour MPs representing Leave seats that they want a deal – but they don't want to stick their neck on the line just to reduce May's defeat from historic to just pretty bad. A strengthening of workers' rights in deal and a stronger town funds have been parts of the government's effort to win these MPs around.
The only way this deal will pass is if everything falls into place which tends not to happen when this Prime Minister is involved. That said, May allies will be fairly content if they can simply reduce the size of defeat significantly. There's a view that these changes offer those looking for one a ladder to climb down – but won't be enough to convince those who are content voting the deal down. Were they to succeed in doing this, the vote on Wednesday on no deal and then on an Article 50 extension would most likely still happen. But May could still argue that her deal was the only show in town – try and stop Parliament taking control of the negotiations and potentially go for a Meaningful Vote 3.