Our prime minister lies, not just a bit but a lot. All of us lie, at least a bit, sometimes – but not serially and not from the office of prime minister. This matters because it debases not just politics but democracy. Eventually it’s not just party advantage or national credibility at stake, but lives.

The campaigner Peter Stefanovic collected some of the lies and put them in a video that has been watched 11 million times. Green MP Caroline Lucas picked up on this and organised a letter for party leaders to sign calling for the Privileges Committees to conduct an inquiry into the PM’s behaviour. The Lib Dems, the SNP, Plaid Cymru, the SDLP and the Alliance all signed alongside the Greens.

Keir Starmer was asked not just to sign the letter but, I am told, to lead the campaign. He refused. A spokesperson told the Guardian that ‘Labour did not normally sign up to initiatives launched by other parties’.

Now, I’m a serial proponent of a Progressive Alliance, one that would see parties collaborate to beat the Tories and pass legislation for proportional representation – plus a minimum viable policy programme to address the economic and social impacts of a post-Covid world and the climate disaster. So, I have form on this issue.

But stand back and try and coolly analyse the approach of Labour here. The country and its democracy are being fatally undermined by lies at the very top of government – which, in part at least, is shoring up support for said lying government. This is partly is about winning but more importantly about ensuring we live and are governed by at least the semblance of proper democratic practice.

Labour’s response is therefore mystifying on two levels. First, no party owns the ‘democracy issue’ or the rules and behaviours that govern it. There should be no party advantage here – just a deep and abiding desire to do the right thing. In this instance, the right thing was to join with others and point out the alarming drift towards misrepresentation of the truth, and how all of us lose out as a consequence.

The second point is more nakedly party political. For Labour to flourish, as it did before its victory in 1997, there needs to be a general mood that it’s time for a change, and that the Tories are out of touch and have become too comfortable with unchallenged office. Labour cannot create this mood alone. It needs to work with others and be seen to do so. Labour was asked not just to sign this demand – but to lead it. It refused. Compare and contrast how New Labour led the charge against Tory sleaze in the 1990s – it worked with others, even to the extent of agreeing, along with the other progressive parties, to stand aside in the Tatton constituency to defeat a Tory MP mired in sleaze. It worked.

At the next general election Labour needs to win 124 seats for a majority of one. At the moment the polls suggest it may lose even more seats. Unless there is an earthquake no one can see coming Labour is staring down the barrel of a fifth straight general election loss. That has never happened before. It increasingly looks like the only way Labour can get back into office is via a Progressive Alliance that sees it cooperating with other parties. That can’t come at the last minute, but must be built up over time, as it was in the early 1990s when Labour worked with the Liberal Democrats: witness the Labour MP Robin Cook and Lib Dem MP Robert Maclennan working on democratic reform together.

Now, just think about the 80 seats where the Lib Dems are second to the Tories and Labour stands no chance of winning. Labour needs the Lib Dems to do well in all those places and to encourage Labour voters to vote tactically. And think too of the growing army of voters who are backing the Greens – Labour will need to incentivise them and Lib Dems to vote for Labour in their 124 target seats. This will happen when they see leaders sensibly working together on issues of national interest.

Labour’s refusal to cooperate is both sad and self-defeating. The party of solidarity shows none to people who are campaign for the same better politics. Labour has to decide whether to work with others or most likely lose again alone; whether to share some power or end up with none. But of course, it’s not just Labour that loses out but a country and people who need a change of government, a new politics and a new and good society.

Neal Lawson is Director of Compass. Find out more about Compass’s campaign for a Progressive Alliance here.