For months political authority has been draining away from Theresa May. On Wednesday what remained of Conservative patience with a failing leader was visibly withdrawn as the prime minister stood in the House of Commons announcing an EU withdrawal agreement bill (Wab) that many of her own MPs have already rejected. Senior cabinet ministers were absent; the government benches were not packed as they usually are on such occasions, and attendance thinned out quickly.

The Tory party that had backed Mrs May in a confidence ballot last December appeared to be voting with its feet against her. Andrea Leadsom, leader of the Commons, quit the cabinet saying she could no longer endorse the prime minister’s Brexit legislation. Other MPs and ministers openly speculated about the manner of Mrs May’s departure and plotted to accelerate it. The debate acquired a valedictory tone, as if those who remained in the chamber recognised that this was a closing scene in the final act of Mrs May’s time in office.

The prime minister’s debilitation has countless causes dating back to choices she made within months of her arrival in Downing Street. But a definitive provocation to her party was the presentation of a “new deal” to deliver Brexit that was no such thing. The Wab is a legislative device to enact the same deal that was agreed in Brussels last November. The reasons that a majority of MPs reject it have not changed since then.

Features of the bill that the prime minister has presented as innovations are, in reality, political scaffolding intended to uphold a structure that has already collapsed. Mrs May has scattered notional concessions to different parliamentary factions with a view to assembling a cross-party coalition to pass the same old deal. This enterprise was doomed to fail for the same reason that Mrs May’s entire period in office can be deemed a failure: the political and diplomatic foundations required to assemble a majority for any Brexit model were not laid.

From the initial rejection of the single market – on the basis that free movement must at all costs be ended – Mrs May has prioritised personal prejudices and narrow party interest over rational evaluation of Britain’s strategic and economic interests. When she lost her parliamentary majority she relied on ultra-partisan reinforcement from the Democratic Unionists, who cannot claim even to represent the majority will of Northern Irish voters. The prime minister did not cultivate relationships with continental counterparts that might have engendered trust and flexibility in talks. She never rebuked or repudiated those in her party who spoke of the European project with venomous animosity, but instead allowed her own negotiating mandate to be curtailed by their reckless, impossible demands. In so doing she made herself – and, by extension, her country – look like an unreliable and sometimes ridiculous actor on the international stage.

Only when all of her political capital had been spent and her deal trounced in parliament did Mrs May show an interest in accommodating the views of more moderate Eurosceptics and former remainers. By the time she was prepared to speak the language of compromise her capacity to deliver it had shrivelled to nothing. Any compromise she might have brokered could only be cosmetic, because the substance of the withdrawal agreement is non-negotiable and the contest to replace her as Tory party leader is already effectively under way. Mrs May could not bind the hands of a successor, and Labour would not trust any of the candidates to honour commitments she had made.

The Wab is a prop in a parochial Westminster power play that ends, sooner or later, in the prime minister’s resignation. When she speaks of a “deal” she is talking about a domestic pretence of agreement to defer a vast array of difficult questions about the UK’s future relationship with the EU. Mrs May has lacked the courage and vision to articulate compelling answers to those questions, still less to persuade the public and parliament to trust her judgment. That profound and protracted failure is in large part the reason why the whole Brexit process has broken down, why British voters will participate in European parliamentary elections on Thursday, and why that poll is certain to administer a brutal verdict on the prime minister and her party.