Ed Miliband’s cause wasn’t helped, in Labour’s last leadership election, by winning fewer votes than his brother among the party’s MPs.  That was under Labour’s old rules for these polls, in which MPs plus MEPs, constituency parties and the trade unions plus affiliated sections had a third of the vote each.  These were swept away after the Collins Report of 2008, itself triggered by the row over Unite’s vote-manipulating activities in Falkirk.  Labour now has a one-man-one-vote system instead: sounds fine and dandy.

But think on.  If Miliband was hobbled from the start by gaining the support of under half of Labour’s MPs (and that, remember, was in the final round of voting: in the first, he won only a third), how on earth can Jeremy Corbyn hope to flourish – or even survive in post – if he wins this time?  Only a sliver of his colleagues back him.  Indeed, he’s only in the contest at all because a few “morons” – to borrow the self-lacerating phrase of Margaret Beckett – voted to put him in it, for either sentimental or tactical reasons.  Some of the latter seem to have had a Baldrick-type “cunning plan” to boost Andy Burnham by putting Corbyn in the poll to gain transfers. A fat lot of good it is doing their man.

There is a moral here for our own Party, which will undertake its own contest before 2020, perhaps as soon as next year.  It is that the next Conservative leader is also unlikely to prosper if he or she doesn’t have the support of at least a plurality of the parliamentary party.  The presumption of Party members should thus be to vote for the person who has won the most backing from Tory MPs.

I am not saying that it should always be put into effect.  Tim Montgomerie made this site’s name, in part, during the 2005 contest by campaigning to keep members’ entitlement to vote.  It may sometimes be right for them to defy the Parliamentary Party.  I would argue that they were correct in 2001 not to return the man who topped the MPs’ ballot, Ken Clarke.  But they would usually be wise not to take a different course.  There would probably have been turmoil in 2005 had they not backed David Cameron.  And the candidate who they preferred to Clarke – Iain Duncan Smith – never recovered from having the backing of less than a third of his colleagues.

Unlike most party members, MPs see their leader up close and personal.  That doesn’t mean the view they take is always right: far from it.  But their vantage helps to demonstrate that our political system is a Parliamentary system. Corbyn’s left-wingery is not an automatic bar to him winning in 2020.  But if Labour MPs won’t tolerate him, he’ll never even get there.  Today, it is claimed that two-thirds of front bench Commons won’t be filled if he wins the leadership election.  This is Labour’s biggest Corbyn problem of all.  When the time comes, we mustn’t make the same mistakes.