Normally, an opposition could enjoy a week like the one that’s coming. It could sit back, relax and break open the popcorn as Theresa May walks into a Commons defeat on the policy that has defined her premiership. It could delight in yet more days of debate in which next to no one on the prime minister’s own benches rises to speak up for her, savouring the sight of a governing party that is devouring itself in full public view.

But these are not normal times. For one thing, the stakes are too high. It isn’t just a regular piece of domestic legislation that the government is foundering on, but the most important – and potentially most damaging – move in the country’s postwar history. Labour cannot be passive in this process, giving the government enough rope to hang itself. Because if this goes wrong, it’s not just May’s career or the Tories’ electoral prospects that are wrecked: it’s the country.

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It’s not only an abstract sense of national duty that should be weighing on Labour: the more concrete fact is that in a hung parliament, an opposition has genuine power. With a substantial number of Conservatives having slipped the bonds of party loyalty, ready to vote with the opposition if they have to, Labour can shape the course of Brexit. The downside is that if this goes badly, it won’t be just be May and the Tories who are blamed: Jeremy Corbyn and Labour will be in the dock, too.

All of which means the era of “constructive ambiguity” has to end next week. Maybe it worked for a while, this Labour blend of remain and leave, wording every pronouncement on Brexit in language bland enough to go down easily in both Hampstead and Hartlepool. But that fudge is past its sell-by date. We are at the moment of decision now.

Not that you’d know it from the noises the party is still making. Take the article in the Guardian on Friday by Corbyn himself. It spoke repeatedly of Labour’s “alternative plan” for Brexit, by which Britain would have all the benefits of the single market – such as “frictionless trade” – with none of the unwanted costs. This is a plan in the same sense that I “plan” to win the men’s singles title at Wimbledon next year. It is not a plan at all, but a desire for something that is demonstrably out of reach.

The EU could not have been clearer or more unbending on this point since the vote in 2016 (and long before). The only way to get frictionless trade with the single market is not by forging “a new and strong relationship” with it, as Corbyn writes, but by being in it, as we are now. Equally, you cannot hope to leave the EU and simultaneously retain “a British say in future trade deals” done by the EU. To argue otherwise is to offer the same cake-and-eat-it, unicorn-filled impossibilism peddled by Boris Johnson and Jacob Rees-Mogg. That it comes with a red rosette on its lapel does not make it any less dishonest. Which is why even one of Corbyn’s most senior shadow cabinet colleagues told me the leader’s article was “poor”, while its central claim – that there is some new, alternative, backstop-free deal just waiting to be negotiated with Brussels by Labour – is “nonsense”.

To my mind that hope was always absurd, its impossibility captured by that meme of May on the phone, captioned: “Hello, is that Sky? Yes, I’d like to cancel my subscription but still receive all your channels.” Shadow ministers admit that is even more unarguably true now. Beyond “some tweaks to the political declaration”, there is nothing more the EU would offer a freshly installed Labour government, even if, as Shami Chakrabarti insists, “there’s such a positive atmosphere” the moment Corbyn walks into a room of European leaders. The EU27 are not going to pretend the last two years of talks didn’t happen and start all over again.

Which is why the first move Labour promises to make, once May’s deal is voted down, is also hollow. It will call for a general election, arguing that that is the best and most legitimate way for the country to resolve its Brexit conundrum. But how would an election resolve it exactly? If Labour simply offers an impossible alternative to the May deal – a pain-free, “jobs first” Brexit in which we get all the pleasures of EU membership and none of the pain – then even if Labour were to win, the question would not be settled, because it would be as unable to deliver its Brexit as May was hers.

That leaves two options: Norway-plus or a second referendum. Labour won’t be able to obfuscate its way through that choice for long, though it might try. So far the signals from Labour’s top table are hostile to Norway: because it would make Britain a rule-taker and, though they don’t like to say this too loudly lest it sound as if they’re anti-immigration, because it would commit Britain to freedom of movement. (Codedly, Corbyn signalled his opposition to free movement in his Guardian piece by looking forward to Labour “setting migration policies to meet the needs of the economy”, which is a right-on way of saying: no blanket right for EU citizens to come here.)

So a people’s vote could be the last option standing. After many months, Keir Starmer and John McDonnell now seem to agree on that, backed by Tom Watson. Ranged against them are Unite’s Len McCluskey – though McCluskey’s members back a second referendum – and the leader’s office, including the pivotal figures of Seumas Milne and Karie Murphy. Corbyn himself is said by one insider, who is not hostile to the leader, to be “pretty disengaged”.

If Labour does end up calling for a people’s vote and getting it, plenty will praise the party for its strategic acumen. It will have played the long game, kept Labour leavers onside at the 2017 general election and picked the right moment to make its move. But then there will be a new question: how to win that second referendum for remain? How much easier would that task have been if Labour had spent the last two and a half years exposing the Brexiteers’ project as the impossible fantasy it is – rather than indulging it, accommodating it and even echoing it, right until the very last moment?

• Jonathan Freedland is a Guardian columnist