Most pollwatchers think Corbyn will be somewhere between electoral trouble and a disaster for Labour. To test whether they’re right, what should we look for in the polls?

We expect new leaders to get an initial poll bounce as the public compares them favourably with their predecessor (whose career – like all politicians’ – has just ended in failure) and then for their scores to drift once the negatives come out. I’ve had a look at historical polls (thanks to Mark Pack’s spreadsheet of every poll) to see how long this bounce lasts.

From the first 12 months of polls of each post-war Labour leader’s tenure, there are a few interesting results:

1) Nearly every Labour leader had a boost in the three months after they took over.

2) A few (Gaitskell, Wilson, Miliband) don’t seem to have changed views much: they got a small bounce when they took over, which gently unwound over the year.

3) Nearly all the ones that had a dramatic bounce (Smith, Blair, Kinnock) lost some of that gain but still had most of it after a year…

4) …but the ones that lost support in their first three months (Callaghan, Foot) lost even more before the end of the year (the latter comparison could be bad for Corbyn).

5) Every Labour leader who started with support below 40% immediately increased it by 5pts or more (except Miliband).

6) Brown really should have called that election in 2007.

Clearly Corbyn’s a special case: Labour have never had a leader like him. Perhaps there’s not so much to learn from this.

But if there are any parallels, I would say anything less than a 5pt boost by Christmas (ie if Labour’s on less than 36%) would be worrying, and support at Christmas lower than where it is now (around 31%) would point towards further loss of support over the coming nine months.

And if Labour sees a major poll boost in the next three months, the following three to six would suggest whether Corbyn’s looking more like a Brown or a (the irony) Blair.