This morning the newly elected Mayor of London Sadiq Khan said that, in order to win a general election, Labour needed to become a broader church, appealing outside its core support.
He's right of course, and what he said this morning is a fairly clear attack on Jeremy Corbyn's political strategy, which is very much aimed at appealing to a fairly narrow set of voters. Cprbyn's supporters have jumped on last week's results as a demonstration that, at the ballot box, a Corbyn lead Labour party does much better than people predict. That's true insofar as they did do better than many predicted (albeit they didn't do well), but if last Thursday had been a General Election they would have done much, much worse. Here's why.
We know from Labour's membership figures and the rise of the Momentum group that Corbyn has been very effective at motivating those people who do agree with him to an unprecedented level of political engagement. For every one of the 200,000 or so members who have joined Labour since the last election there will be another ten people who haven't gone so far as joining but have been, to some extent, "activated" by Corbyn. Politically activated people, as you would expect, are much more likely to vote than the average. That means that in low turnout elections like the ones last week (in many places the turnout will be half that of a General Election) those people form a disproportionately high segment of those who vote. I've no doubt that in some of the places where Labour were expected to badly and didn't, these people (many of whom wouldn't normally vote in a local election) had an big effect on the outcome.
What I'm going to say next is somewhat obvious. In a General Election the turnout is much higher. Literally millions of people who pay little attention to party politics in the intervening five years come out and vote, largely on the basis of who they want to be Prime Minister. However well motivated Corbyn's army is, in a General Election they'll simply be washed away like a sandcastle by a tsunami of the generally middle class, slightly right of centre voters who have decided more or less every General Election since at least the 1980s. They stuck with Labour (more or less) from 1997 to 2005, a load of them then dabbled with the Lib Dems in 2010 before shifting back to the Tories in 2015, driven by the fear of the SNP and Ed Miliband.
Corbyn's central thesis, that you can outnumber this group by building an alternative movement of mainly young & working class non-votes, is total bollocks.
I don't mean that as a political statement, it's just maths. Even if you did motivate every single one of those people, there just aren't enough of them to make the difference in a General Election.
So when Corbyn's supporters point to what happened this week as evidence that his strategy is "working" they're being hugely ignorant of quite a simple electoral reality: turnout matters.