The first benchmark for today’s vote is 41 Conservative MPs. That is (as we have explained before) both the largest Conservative rebellion by government MPs against Europe ever AND the largest Conservative rebellion so far during this Parliament. Should at least 42 Conservative MPs rebel, then this will be the largest Conservative euro revolt ever. Forty-two is also the answer to life, the universe and everything, although we don’t expect the whips will see it like that.
The next benchmark is the largest Labour euro rebellion. That occurred in January 1978, when 80 Labour MPs voted against a programme motion for the European Assembly Elections Bill. (There have been bigger splits amongst the PLP over Europe during the post-war era, such as the split over the issue of continued membership in 1975, but these were on explicitly free votes). So anything involving 81 Conservative MPs or more, and today’s vote can be seen as the biggest rebellion against the whip on a European issue by members of any British political party.
The largest Conservative rebellion of all in the post-war era occurred in 1996, when 95 government MPs voted against their whip over the post-Dunblane gun control legislation. So if there are 96 or more Conservative rebels today, this will be the biggest Conservative revolt of the post-war era.
The largest Labour rebellion in the post-war era occurred in March 2003, over the Iraq war. Then, 139 government MP voted against their whip. But all the evidence is that this was not just the largest post-war rebellion, but the largest of any party, on any issue, since the vote over the Corn Laws in the 1840s. So if we get up to that level (and we don’t for a minute think we will) then this is the largest backbench rebellion since the formation of modern political parties in the UK.
Note that these figures exclude abstentions – which are impossible to measure systematically. There’s a good book dealing with all of this, you know.