The denouement of the Brexit epic finds Remainers and Leavers joined in strange unity. Both have vowed to defeat Theresa May’s deal next Tuesday; both insist that this will serve their interests.

Leavers hope to advance an alternative plan (or embrace the apocalypse of no-deal), Remainers hope to secure a second referendum, a general election or a Norway-style agreement.They cannot both be right.

During yesterday’s Brexit debate, Boris Johnson declared that the UK may be “1-0 down” against the EU but “can win 2-0”. This was merely the latest sign that the former foreign secretary has forgotten Lyndon Johnson’s first rule of politics: “Its practitioners need to be able to count.”

Last month the Brexiteers – who hope to take over the most complex negotiation in post-war history – discovered that they did not have the numbers to remove May as Conservative leader. Yesterday they learned that they don’t have the numbers in parliament either. By 321 votes to 299, MPs backed Dominic Grieve’s amendment ensuring that they will have the right to amend the motion that the government will bring forward if May’s plan is defeated.

The nascent “soft Brexit” majority that was created after the 2017 election is now exerting itself. Leavers protest, as May once did, that “nothing has changed”. Steve Baker, the European Research Group’s chief consigliere, stated: “Whatever the outcome of the amendment, it is not legally binding on the Prime Minister. Acts are law, motions are motions. The executive still decides how to proceed.”

This is technically true but politically naive. When David Cameron was defeated over military action in Syria in 2013 he maintained the formal power to take the UK to war. But such was the political force of the vote that he could not credibly do so. Similarly, a prime minister as enfeebled as May could not deliver a no-deal Brexit – an act of national self-harm – on the grounds that there is no legal alternative. If there isn’t now, there will be soon.

And though there is a plausible majority to soften Brexit, there is not one to harden it. The EU will never accept, as some Leavers claim, the abandonment of “the backstop” (which would keep the UK in a customs union in the absence of a solution to the Irish border problem). But even were it to do so, parliament would reject any withdrawal agreement that created a hard Irish border. The Brexiteer dream of “Empire 2.0” – a swashbuckling Britannia striking trade deals with the Anglosphere – is being thwarted by the legacy of Empire 1.0.

The Brexiteers, then, must face the grim truth: May’s deal is a bad option but there is no better one available. Their true quarrel is not with the Prime Minister but with reality.

An increasing number of Brexiteers now lament that May’s proposed deal is worse even than EU membership but the delusion was to believe that it could ever be superior. A soft Brexit would sacrifice political sovereignty – with the UK becoming a rule-taker, rather than a rule-maker – while a hard Brexit would sacrifice economic prosperity.

May’s deal involves elements of both – an imperfect compromise that was both predictable and predicted. Sincere Brexiteers can nevertheless support it on the grounds that it would definitively end the UK’s EU membership and deliver on the result of the largest democratic exercise in British history. Leavers who are unable to support it – and who even concede that EU membership would be preferable – should accept their own logic and back Remain.

But far from doing so, they assert that the revolution has been betrayed and that Brexit is failing because it has not been tried. What they crave, as ever, is the prerogative of the harlot: power without responsibility.