What will defeat on tonight’s meaningful vote mean for the government?

A defeat of around 50 votes would look very different to one of 100 or more.

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Defeat in today’s meaningful vote on the Withdrawal Agreement is inevitable for Theresa May — but its consequences are not. Just what this evening’s loss for the government will mean — both for Brexit and the Prime Minister — depends to a large extent on its shape, and which MPs inflict it.

Downing Street is acutely aware of this, and its efforts as the vote approaches have been focused on restricting to a politically manageable level, rather than overturning completely the margin of defeat. 

Doing so will mean not only convincing many of more than 100 Conservative MPs who have declared against the Withdrawal Agreement, but also a sizeable number MPs from Labour – a party whose whip obliges a vote against the deal, and whose support has not yet been forthcoming in significant numbers. 

The most pessimistic projections suggest that task is both beyond the Prime Minister and in any case futile. Sky News predicts that MPs will vote 424-198: a margin of 226. That would mean a clear majority of Conservative backbenchers were against the deal, as well as most of Labour and the DUP. 

This scenario would dwarf the biggest parliamentary defeat of modern times, which saw Ramsay MacDonald defeated by 166 votes in October 1924 (over a procedural question regarding the establishment of a select committee). It would also mean a similar number of Conservative rebels to the 139 Labour MPs who rebelled over the Iraq War in 2003.

The political message from such a catastrophic defeat would be straightforwardly grim: it would impress upon Brussels that a negotiated settlement of any kind, especially a revised version of the current Withdrawal Agreement and Political Declaration on the future relationship, would have no chance of securing Westminster’s approval. It would also reflect unpopularity among MPs so abject that it is unlikely that it could form the basis for a successful second vote. This logic holds for any defeat of more than three figures

A defeat contained to fewer than 100 votes would be slightly less humiliating for Downing Street, but crucially would also offer a glimmer of hope as far as passing the Withdrawal Agreement in its current form goes. 

Should May manage to reduce the margin to fewer than 95 (the number of Conservative MPs who rebelled on plans to introduce controls on handguns in 1997), 91 (the number of Tory MPs who rebelled on Lords reform in 2012) or 81 (the number who rebelled to vote for an EU referendum in 2011), then she will have managed to convince a significant minority of Tory backbenchers to change their mind on the deal without significant changes to it (or, less likely, via amendments promising further revision). Her argument that rejecting her Brexit risks no Brexit will have had limited but nonetheless significant cut-through. 

A group of up to 20 Labour MPs will have also voted for her deal in this scenario – but they will be limited to those who either voted Leave in 2015, or those who have defied their party whip to back a harder form of Brexit than currently proposed by their party. This is the sort of territory that would convince May that her deal was very much still a going concern – but doubts would remain in Brussels over whether they could offer anything to salvage a victory. 

Anything substantially less than this – a defeat of around 50 or less – is on current evidence completely unrealistic. It is nonetheless the sort of defeat that both the Prime Minister and those MPs loyal to her will be hoping for. Such a margin would mean the Conservative rebellion had been limited to the most hardline Eurosceptics for whom most negotiated Brexits would be insufficiently hard, pure or politically expedient, as well as the DUP. There would be a significant number of Labour rebels too, stretching beyond those 15-20 usual suspects.

Downing Street loyalists believe this is the sort of margin that would signal to Brussels that parliamentary approval would be easily attainable with the right sorts of concessions, particularly on the Irish backstop. This outcome, however, remains as unlikely as victory itself. 

Patrick Maguire is the New Statesman's political correspondent.