One of the things for which Boris Johnson was wrongly mocked was writing two articles about Brexit, one in favour and one against. It was the sort of intellectually rigorous exercise that we should applaud in our leaders, even if we did not agree with the conclusion he reached.

Let us apply the same method to the issue that threatens to divide the Labour Party at its annual conference in September: the progressive alliance. Many members want the party to come out in favour of a proportional voting system and a pact with other non-Conservative parties. I am sceptical, but setting out the arguments for and against is a good way to test them.

The case in favour goes like this. Every opinion poll you look at shows the Conservatives on less than 50 per cent of the vote while parties to the “left” of the Tories together have more than 50 per cent. That was true at the last election, and it is still true now, even if Labour is trailing far behind the government. Even if you add the Reform Party’s sliver of support to the Tory total and leave the Scottish National Party out of the “progressive” side, the progressives still have more support.

So our electoral system is unfair and does not reflect people’s true preferences. If we had a proportional system, the Tories would be denied a majority and Labour could lead a progressive coalition. Changing the voting system can be achieved only in government: therefore, the non-Tory parties must stand aside in each other’s favour to mobilise the progressive majority under the first-past-the-post system to secure a majority in parliament.

Then all we have to do is fight the next election — as separate parties but in broad agreement that the Tories are bad and that we want more public spending on schools and hospitals, and net-zero carbon — and wait for Keir Starmer to lead us to a Dutch-Scandinavian future of permanent progressive coalition government.

The final argument, made this week by Nick Cohen, the Observer columnist, is that Labour has no choice because it cannot win on its own: it has lost Scotland and it starts from too far behind.

The case against is that the idea of an anti-Tory majority is a mirage. We saw what happened when the Lib Dems faced a choice in a hung parliament in 2010. We know that Lib Dem voters may currently lean in Labour’s favour, but a large proportion of them do not.

An electoral pact could make a difference: there are about 10 seats that the Lib Dems could have won from the Tories if Labour had stood down at the last election, and perhaps about six seats that Labour could have won from the Tories if the Lib Dem vote had split 60-40 in Labour’s favour. As David Cowling, the former BBC elections analyst, comments in his study of second-preference votes in police and crime commissioner elections: “Lib Dem voters seem less reliable as foot soldiers in the Great March to Progressivism than either Labour or Green voters.”

As ever, I come back to the article Tony Blair wrote for the New Statesman in 1987, when Labour had just lost another election and was displacing its trauma in the quest for pacts and electoral reform: “There is no decision that would be justifiable for Labour to make in order to win power in a coalition that it should not be making anyway for itself.” Or, as Daniel Shearer argued this week, “there already is a progressive alliance: it’s called the Labour Party”.