I appeared on Radio 4 with Shirley Williams recently and as we were leaving I asked her if she thought Labour might split if Jeremy Corbyn were re-elected. Would the history of the SDP, which she helped set up in 1981, put off Labour moderates from trying something similar?

She thought it might, but suggested an alternative, which was a ‘non-aggression pact’ between all the left-of-centre parties. ‘We can unite around the issues we agree on and get the Tories out,’ she said. I didn’t have time to explore this in detail, but I think she meant some kind of tactical voting alliance whereby supporters of Labour, the Lib Dems, the Greens — possibly even the Welsh and Scottish Nationalists — would agree to vote for the left-of-centre candidate in their constituency who had the best hope of defeating the Tory candidate at the next election.

I’m interested in this idea because I proposed something similar in 2014, except what I had in mind was an anti-Labour pact. It seems preposterous now, but back then I thought there was a real danger that Ed Miliband would end up as our next prime minister and launched a ‘Unite the Right’ campaign to try to avoid this ghastly prospect. The plan was to persuade Conservative and Ukip supporters to put aside their differences and vote for whichever right-of--centre-candidate had the best hope of winning in each seat.

After about a year of trying to get this campaign off the ground I-concluded it wasn’t going to work and I suspect an anti-Tory alliance would founder for the same reasons.

The best counter-argument, which was made to me by several senior figures in the Conservative party, was that a formal alliance with Ukip would be so toxic that any votes the party gained on its right flank would be more than offset by losses in the centre ground. The same objection could equally well be made by the Greens or the Lib Dems to any proposed alliance with Labour while Corbyn remains at the helm.

My response to this was that I wasn’t proposing a formal pact, but rather an informal one. A formal arrangement, whereby the two different parties actually stood down candidates in favour of their rivals, was a non-starter for a variety of-reasons, not least that it would be virtually impossible to persuade local candidates and activists to go along with it. What I had in mind was an alliance at grassroots level, whereby casual supporters of both parties would agree to vote tactically to keep Labour out. Not only would that bypass local candidates and activists, but the leaders of the Conservative party could justifiably claim it had nothing to do with them and thereby avoid the toxification of the party’s brand.

But here I encountered-another problem — which was that in order to build a grassroots movement I needed to persuade supporters of both parties that they had an equal amount to gain. And the difficulty here was that in 2010 there had been far fewer seats in which Ukip came second to Labour than those in which the Conservatives came second. That meant a tactical voting alliance would help the Tories far more than Ukip. The same problem would beset any attempt to forge a pact between Labour and the Greens, whether formal or informal, since the Greens came second in just four constituencies in 2015, all of which were won by Labour. Would Green party supporters really agree to vote Labour everywhere else in return for Labour supporters agreeing to vote for Caroline Lucas in Brighton Pavilion, the Greens’ only seat? Almost certainly not. But Lucas couldn’t make a credible case for-supporting the Green candidate in any other constituency because in every case they would never be in a better position to defeat the Conservative candidate than either the Labour or Lib Dem candidate.

Even if these problems were surmountable, which they aren’t, there’s an even bigger obstacle, which is that many people feel a tribal connection to the party they vote for, and loathe all their rivals. Despite the repeated claims at the Labour conference, there isn’t far more that unites the left than divides it. On the contrary, most left-wing voters hate their internecine rivals even more than they hate the Tories. I’m afraid that what Freud called the-narcissism of small differences means an anti-Conservative alliance of the kind Shirley Williams has in mind is a complete non-starter.

I proposed something similar in 2014, except what I had in mind was an anti-Labour pact

Toby Young is associate editor of The Spectator.