With the departure of David Davis, the Conservative strategy for Brexit has died. Despite embodying a very clear strand of thinking among old-style Tory voters, it’s not the man himself who matters. The simple fact is: Theresa May cannot now get her Chequers plan through the Commons.

The implications of this must be stared squarely in the face:

– The EU27 will regard the Chequers request – for a customs arrangement, voluntary alignment etc – as irrelevant; any support that was building for positive engagement with May’s request will be easily overridden by Emmanuel Macron, who is pushing for the indivisibility of the single market rules.

– May will likely be overthrown before the Tory conference in October. The “second vote” that decides the fate of Brexit will be another snap general election.

– The real alternatives are a Canada-style free trade deal and a Norway-style deal that puts the UK inside both the customs union and the single market.

– Labour must recalibrate its position accordingly.

Before coming to Labour, it has to be recorded that this is the abject failure of a political strategy. It failed in stages: first, when May botched the election and lost her majority. Next, when she spelled out “red lines” at Lancaster House, only to row back and request off-peak membership of the EU at the Mansion House. Chequers was a retreat further, and has finally triggered the breakup of the Tory majority in the Commons.

If the government had made Brexit strategy bipartisan in 2016, placing it in the hands of a Commons committee, guided by evidence from civil servants, there would have been enough moral force behind the Brexit decision, and disarray among progressives, to have set out the ask, then trigger Article 50, enter swift negotiations and secure a deal.

The crucial point is: in all hypothetical circumstances imaginable, Britain’s negotiating position would have been stronger than it is now.

May, Davis and the civil servants advising them ought to be brought before a committee and grilled, with all documents published, to find out if this constitutes misconduct in public office. I don’t really care who replaces Davis, or May: the Tory party can’t have a leadership election without squarely addressing the question: closer to Europe or closer to Trump? It will split over the issue.

When the facts change, you have to ask whether it is appropriate to change your mind. Labour has “won” the political battle that opened up in June 2016. By standing firm on the implementation of the referendum result; pushing consistently for a soft-Brexit deal short of Norway; refusing to support or rule out a second referendum; opening strong communication lines with Brussels and the European capitals, Jeremy Corbyn and Keir Starmer have forced May into a position she cannot carry with her own party.

But in the process, Labour has sacrificed its appeal among the FBPE crowd; people so viscerally opposed to Brexit that when Labour’s Rosena Allin-Khan demanded publicly-funded screenings of the World Cup semi-final on Saturday, the FBPE-ers denounced her for creating a “distraction” from Brexit.

One of the core conceits of FBPE is that “Labour supports the Tory hard Brexit”. In the past months this has been pushed as a form of voter-suppression tactic (like Trump’s “Hillary is a racist” tactic in black states). It has always been a lie.

But as of today, there is no Tory hard Brexit. There is political convergence towards a soft Brexit and whoever replaces May will be forced further towards either single market membership or alignment.

However, while maintaining Labour’s position of respecting the referendum result, we have to concede that the “stop Brexit” position is also strengthened by May’s crisis, and on two grounds.

First, it may be that there is no outcome to Brexit that is in the national interest. Let’s unpack this, as Barry Gardiner did yesterday on the Sunday Politics. Economically, Brexit was always going to damage the UK; but its civil society and democracy might be irreparably damaged by an elite-manoeuvred sabotage of Brexit.

Since the very forces trying to do Brexit have now thrown up their hands in disarray, the political case for a rethink is stronger. The Tommy Robinson crew will riot if Brexit is cancelled; Nigel Farage will “re-enter politics” and the Tories will split – none of these are a catastrophe for the country.

Having watched their chosen party, the Tories, try and fail to do a hard-ish Brexit, any government – including a Tory replacement for May’s cabinet – could legitimately say it is impossible to implement the 2016 result without damaging the national interest.

The second grounds are constitutional. Only a vote in parliament could trigger Article 50. That was the outcome of the Miller case. Only a vote in parliament can make legal any deal the government does. And only a vote in parliament could allow for a second referendum, once the deal has parliamentary approval.

For all the fury, the actual disagreement between the Labour frontbench and many FBPE-ers was about sequence and principle. This may now change – and for a reason to do with how this plays among grassroots Tory and pro-Leave voters.

Boris Johnson is said to have concluded that staying in the EU might be better than a half-cocked, rule-taking deal. As that thought percolates through the golf club bars, among the yachters and the well-off British families in their Mediterranean villas, the liberal part of the Tory demos may now fall into line with the thinking of Siemens, Airbus, BMW and Honda – that the entire situation is shit and needs drastic reversal.

So here’s what I want Labour to do, since it’s now clear it could form a government this autumn

1. Vote against, on a three-line whip, all attempts to codify the Chequers position in the Commons. No more tolerance of the handful of Lexit-voting MPs.

2. Publish a positive outline of both its single market proposal and its proposal on migration. This can’t wait - any attempt to fight a second election without them would be unprincipled.

3. Offer a second referendum on the final deal signed with the EU27, should the party form a government – ie move on from the position of “not ruling it out”.

Any deal would be unlikely to be achieved by the end of March 2019, so alongside this, Labour should retain the option of requesting an extension, citing political chaos and Tory incompetence.

A second referendum, with a Labour majority, plus the SNP, Lib Dems, Plaid Cymru and the Greens behind a Norway-style proposal, would not be a re-run of the Tory fiasco of 2016.

If a Corbyn government could secure a variation on a Norway-style deal, business and the unions would buy it; the xenophobes and Putin puppets could be faced down. In a second referendum, the Electoral Commission and the security and intelligence services would be on guard, not asleep on the job, to guard against law-breaking and foreign manipulation.

The pro-Brexit voters in some Labour heartlands would have to decide if they care more about a tanking economy, closing automobile plants and nursing shortages, or the tub-thumping of Putin-funded xenophobes. The entire weight of a revived Labour machine could be put behind convincing them, in a way that just didn’t happen last time.

The facts have changed and, like Keynes, they might change their minds. If the second referendum took place after Labour legislation to reform media ownership, the poison pens of the tabloids could be balanced by a more diverse national media.

Why am I arguing for a change in emphasis, moving from “not ruling out” a second referendum to promising one, once a Labour government has negotiated a deal? Because the opportunity exists for Labour now to unite the country.

Labour has the chance to unite the diverse factions of progressive Britain behind a clear course of action: vote down the Tory deal, trigger an election, renegotiate the softest possible break (involving parliament, not just the cabinet) and put it to the British people.

An added bonus, for the Labour frontbench, would be that the (already diminishing) momentum for a millionaire-backed centrist party led by “12-20 Labour MPs”, as reported in Skwawkbox last week, would evaporate.