Many people on the left seem oddly relaxed about the fact that the Labour leadership is in negotiations with a Tory government about presenting a joint proposal to deliver Brexit. Perhaps this is because they know something I don’t; that the negotiations will definitely fail, for example. Perhaps this is a dance that neither participant can be seen to break up, while each gazes blankly over the other’s shoulder and waits for the music to end. After all, Theresa May needs something to show to the European council when she asks for her extension.

But this should not be a relaxing situation for the left, and especially not for supporters of Jeremy Corbyn. It is one thing for leftwing leavers to be nominally on the same side as Boris Johnson, or for Labour remainers to march on the same demonstration as Anna Soubry. It is quite another to actively reach out across party lines to deliver Brexit under a Conservative prime minister. If warnings from history can cut through the fog of parliamentary process and summits about summits, perhaps we need to remember Ramsay MacDonald.

This is not 2016. Brexit is not the clearcut will of the people. A majority of voters now oppose it, as do a large majority of Labour supporters. Delivering Brexit means setting yourself against a mass movement, a million of whom have marched and 6 million of whom have petitioned for the outright revocation of article 50. Labour cannot expect to demoralise its activist base by choosing to implement a policy they regard as a fundamental affront to their values, and then just talk about school funding instead.

There is no way that a compromise deal agreed by Corbyn and May could safely guarantee the workers’ rights and social protections currently enshrined in EU law, given that it is the stated policy of most likely future Tory leaders to tear them up and deregulate the economy. It would also betray Labour’s democratically agreed policy, which is to oppose a deal that does not meet its six tests, including that it should deliver “the exact same benefits” as EU membership. There is only one way to fulfil Labour’s political needs: to tie whatever comes out of these talks to a confirmatory public vote.

Without being attached to a public vote, the delivery of a joint Brexit deal would precipitate a deep crisis for the Labour left. Labour would face a wave of frontbench resignations, and could probably not even rely on a majority of its MPs to vote through the deal. Labour’s rank and file, most of whom are now quietly unhappy and hoping for the best, would lose patience and trust. The level of rancour within the party and the left would terminally destabilise and isolate the Corbyn leadership.

The political project headed by Corbyn has an immense bank of political capital. It could spend that capital on completely reforming the Labour party, going much further than simply overturning the erosion of internal democracy from the Blair era. It could put forward an even more radical domestic programme, or a world-leading campaign to combat climate change. To instead spend it all on delivering Brexit alongside May would be a bitter, pointless tragedy.

Perhaps some at the heart of the Labour machine do not see the scale of the danger, or genuinely believe so much in delivering Brexit that they are willing to trade in Corbyn’s leadership for it. Others may think that a clever communications strategy, or quickly changing the subject to the NHS, or staging a separate doomed parliamentary vote on a referendum, will somehow absolve Labour. But to most people’s ears, and when the history books are written, this will all just be noise.

In saying all of this, I and others on the left will be accused of seeking to undermine the current Labour leadership. My response is clear: who are the real loyalists? And why are such a substantial number of people defending, even recommending, a course of action that would so obviously be a total disaster for Corbyn’s Labour party?

• Michael Chessum is a writer and socialist activist