This ought to be a time for the flourishing of big ideas in politics. For those who drove it, Brexit is about reasserting Britain’s imperial role in the world, and stripping out the modest social democratic reforms on workers’ rights and regulatory standards now enshrined in EU law.

Under new leadership, Labour has been presented with an opportunity to mount the concerted, principled defence of immigration and internationalist politics that the party, and the wider left, have been avoiding for decades.

But in the corridors of Westminster, the opposite process is under way. Driven towards the cliff-edge and ground down by the crunching of parliamentary arithmetic, a large proportion of the political class are now desperately triangulating to avert disaster. For some, disaster is a no-deal Brexit, for others it is no Brexit at all. In this context, the prospect of common market 2.0, a soft Norway-style Brexit, will begin to look appealing. And it could pass, if Labour unites behind it.

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There are all kinds of reasons why a Norway-style deal might look tempting for some on Labour’s frontbench. Superficially, it passes the test of delivering the referendum result, while keeping the UK in the customs union and single market, and preserving crucial rights and protections for workers and the environment.

For the group of shadow cabinet ministers and prominent union leaders who have spent months pushing back against the idea of a fresh public vote in spite of party policy, there are sharper, more factional benefits. Soft Brexit would deny their opponents on all wings of the party, and in the grassroots, a victory.

But it would be a profound mistake for Labour to go down this path. Soft Brexit is the least popular policy with the public. It would oblige the UK to take all of the rules and regulations – including the state aid rules so often cited as problematic by pro-Brexit figures on the Labour left – while abandoning a seat at the decision-making table. The only people who think that this outcome would “deliver the result of the referendum” are remain voters desperately attempting to triangulate out of the situation.

The common market 2.0 position has been sculpted carefully by a cross-party group of former grandees from both main parties. It is not backed by anything like a grassroots movement. Unlike the movement against Brexit, it has organised no massive marches, no campaign of motions through party branches, no stalls and door-knocking in the rain. If a Norway-style Brexit deal does eventually win out, it will be because it commands the support of the Westminster bubble. It will neither solve the material problems that caused the Brexit vote, nor satiate anyone who voted for it.

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Things could have been very different. If, in the immediate aftermath of the referendum, Labour had come forward with a plan for Brexit that included single market membership, it could well have won out. Rather than being framed as a messy compromise, this could have been Labour’s bold offer to the 48%, and could have gone alongside an uncompromising defence of free movement. Instead, Labour chose to go with a harder version of Brexit, openly abandoning free movement and only committing to a customs union as late as February 2018. Its position on full single market membership is still not clear.

There is now a mass movement against Brexit. October’s People’s Vote march was the biggest demonstration in Britain since the Iraq war, and it could be eclipsed by a much larger one on 23 March. An army of campaigners, many of them new to politics and instinctively on the left, are leafleting, running stalls and knocking doors all over the country. This is a movement whose demands are supported by the overwhelming majority of Labour’s members, including those on the left – despite the prominence of establishment politicians within the official People’s Vote campaign.

On paper, a Norway-style deal might not look like the worst outcome. Many remainers would be relieved to have close ties to Europe and to retain free movement. But for Labour, delivering a Brexit of any kind will be ruinous. It would demoralise the Labour membership, and hand a stack of ammunition to Jeremy Corbyn’s opponents on the right of the party and in the Independent Group. It would fail the test of delivering the referendum for anyone who voted leave.

For Labour, there is now only one option: to join the mass movement and fight for the big ideas, not the Westminster fudge.

• Michael Chessum is a writer and socialist activist