Polling expert Leo Barasi of the PB/PM podcasts puts his case

Why I think, contrary to the smartest Labour journalists like Stephen Bush, the rules on eligibility for the Labour leadership election help Eagle, not Corbyn:

1. It’s indisputable that pre-2015 members are much more pro-Eagle (or whoever the anti-Corbyn challenger is) than members who’ve joined since May. Eagle wins easily among older members, as YouGov’s latest poll shows.
2. Despite that, the poll has Corbyn beating Eagle 50-40 among all members – which is because 46% of the sample were post-2015 joiners (who are much more pro-Corbyn).
3. If you exclude members who joined 13 Jan onwards – as the rules will – the membership is therefore rebalanced towards people who were members before May 2015. In itself, that helps Eagle.
4. But maybe those who joined in January to July 2016 are actually more anti-Corbyn, which would mean the rules would help the incumbent.
5. According to reports, membership rose over that time, particularly with a reported 100,000 increase since 23 June.
6. Surely this was mostly Corbyn supporters. To dispute that, you’d need to argue that there’s been a surge of opponents of Corbyn joining up January to July, which outnumbers the Momentum people.
7. Basically, I think, in January to July: Pro-Corbyn joiners > Anti-Corbyn joiners   (you could also put Anti-Corbyn leavers on the first side of the equation although that’s a bit more complicated as they’re not directly affected by the rule change, though many would probably have signed up as supporters).
8. The other issue is how many the two sides could have recruited if supporters could be signed up. In principle, this could have greatly helped Eagle, but it depended on engaging outsiders.
9. While it could have been critical, allowing easy access for supporters to sign-up could just have much helped Corbyn. In fact, that seems more likely.
10. The change in eligibility isn’t as big a difference as a May 2015 cut-off would have been, but overall, it feels like the membership will be quite finely balanced now. Supporters might have been the only lifeline for Eagle if there hadn’t been the eligibility ruling – but with that change, I don’t think they’re essential.

Leo Barasi @leobarasi

Leo Barasi writes about public opinion at Noise of the Crowd