We know that May's deal will be defeated tonight. Here's what we don't know

There are two groups worth watching in tonight's votes.

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What we know for certain is that the withdrawal agreement will be defeated again and by a heavy margin (again). Theresa May has secured only token concessions on the Irish border protocol and has offered even less in the search for Labour votes – a non-existent commitment on workers’ rights and a derisory amount of extra money for small towns.

But there are a couple of things to look out for in the vote. The first is the scale of the Labour rebellion. Four MPs elected under a Labour banner in 2017 broke the whip last time and they will do so again. (For ease of reading I am going to count MPs who were elected as Labour in 2017 and have continued to follow the whip even after suspension or voluntary exit as Labour MPs for the purpose of this article.)

There are two pools of Labour votes that might opt to back May’s deal. The first is committed longtime Leavers, who want Brexit but do not want a no-deal Brexit. Two of that group, John Mann and Frank Field, both voted for the deal last time. They could be joined by Kelvin Hopkins, Ronnie Campbell and Dennis Skinner but it is highly unlikely that they will as all four are close political allies of the present Labour leadership. (Kate Hoey and Graham Stringer, the other two Labour Leavers, have both argued for no deal rather than May’s deal.) The other, significantly larger group are Labour MPs who campaigned for Remain but for a variety of reasons are now backing a harder Brexit than the one envisaged by the Labour leadership. Two of that group – Ian Austin and Kevin Barron – voted for the deal last time and they will be joined by at least two more – Caroline Flint and Jim Fitzpatrick.

That number will give us a sense of what the absolute floor of any Labour rebellion on a whip to back a second referendum will be. But the important number concerns the Conservative party and the potential for another election.

The biggest thing standing in the way of another election is that Tory MPs don’t want one: they fear that they would, as one minister put it to me, “disintegrate” over the course of a campaign as they would a) be led by Theresa May and b) have no Brexit policy they could plausibly run on.

If Theresa May can get, say, 250 Conservative MPs to vote for the deal – 196 voted for it last time, while 118 voted against – then it becomes easier to see how the Conservatives could fight another election on Brexit without coming apart at the seams.

And those are the numbers to watch in terms of working out whether it is plausible that MPs could turn to some kind of electoral event to resolve the Brexit crisis.

Stephen Bush is political editor of the New Statesman. His daily briefing, Morning Call, provides a quick and essential guide to domestic and global politics.