Not so long ago the idea of a “progressive majority” was popular amongst leftish intellectuals. They noted that if you added together the poll ratings, and even general election votes, of Labour, the Lib Dems, the Greens and (for some people) the Scots and Welsh Nationalists, there was a clear majority of the electorate, outnumbering the hated Conservatives and Ukip. This majority was regarded as a state of nature, and so, it was argued, proportional representation would lock it into the political system for good.

It was always optimistic. Both Labour and the Lib Dems drew voters who would happily support parties that were not “progressive”. This word is left’s own favoured description for itself. Since, in practice, most “progressives” oppose any kind of reform to make the state or the economy more efficient, I really can’t bear not to place it in quotation marks when it is used to represent the broad left, rather than those who genuinely favour ideas that stand for the positive progress of society..

The flaw in the progressive majority idea is now open for all to see. In the 2015 General Election the Conservatives and Ukip outpolled the “progressive” parties. And that is before any disqualification of the Lib Dems after they entered coalition with the Conservatives. And if that wasn’t enough, the clear majority for Brexit, not supported by the “progressive” parties, confirmed it. Most parliamentary seats for “progressive” parties voted to leave the EU.

This is a fact that the far left (I almost used the word “radical”, but once more the word would be seriously misplaced for a bunch that includes so many people nostalgic for the 1970s). Many are convinced that Labour did so badly in 2015 because it was not left wing enough, with its half-hearted embrace of austerity. Challenged, they suggest that there is an army of disillusioned non-voters who could be drawn into voting for a party of the true left. Certainly there are a number of under-30s that could fit that description, but not enough. In fact most people who explore the polling data suggest people who do not vote are often less-educated and supportive of populist right wing policies. The Brexit referendum was relatively successful in bringing these non-voters out, and they did not vote Remain.

And yet calls for a progressive alliance persist. The Labour left is sceptical, to be sure – to them Labour alone should be the progressive alliance. But many Greens and Lib Dems would contemplate ganging up with other parties in order to push forward progressive reforms. These include constitutional and electoral reform, sustainable economics, and stronger environmental protection. In principle I would support such an alliance, but only with a Labour Party genuinely committed to political pluralism – which rules out the current leadership.

And yet, even if Labour could be brought into the picture, the numbers don’t add up, even if the SNP could be brought into it. An alliance would need to present a serious challenge to the Conservatives in English constituencies. Labour and Green support might help the Lib Dems recover some of their lost seats; the Greens might pick up one or two seats. But it is very hard to see how Green and Lib Dem support would give enough help to Labour. Instead it is more likely that the Tories would successfully exploit Labour muddle to destroy the whole alliance.

So, is it game over for progressive politics? Not quite. Brexit may have won the referendum, but Remain still managed 48% of the vote. But that 48% includes a lot of people who normally vote Conservative. If a way can be found to peel these voters away into a progressive alliance, then it could be back in business.

And it isn’t hard to see what might do this. The Prime Minister Theresa May is enjoying a political honeymoon, but her party is at sixes and sevens over Brexit. It is not at all clear what shape a post Brexit Britain will take because her party is hopelessly divided on it. The moderates want to create a cosy relationship with the EU, in order to protect investment and the economy. But the Eurosceptics that make up so much of the party will not stomach the compromises that entails. Meanwhile, if the British economy goes into recession, as many fear, the pressure on government finances will drive further division. It is not beyond imagination that the only way out will involve a second referendum.

The Conservatives might split under the strain. That is unlikely. As Labour will find out as its MPs contemplate their position, the pressures for the large parties to stay in one unit under Britain’s electoral system are huge. There are no safe seats for breakaway parties, and safe seats are most MPs have no real idea of how to fight a hard seat or the stomach to do it. But the splits will undermine the Tory credibility, giving the chance for other parties to take their votes.

And surely this is the historic mission for the recovering Liberal Democrats. Labour has lost interest in anybody that has contemplated voting Conservative (as both leadership contenders vie to prove how left-wing they are), and the Greens have never had it. There is nobody else to fill the vacant space. For once the coalition experience may prove a positive. That might then revive the idea of a progressive alliance, though the credibility of the Labour Party would be a major obstacle. It remains the best chance for progressive politics.

The Lib Dems are understandably focusing on their core vote, and not on scooping up flighty floating voters. But in order to achieve anything the party will have to return to appealing to these voters in due course. Disillusioned Labour voters will not be enough. The party will have to detach centre-right voters too. That should be food for thought for the party as it tries to redefine itself in British voters’ minds.