Politics is like comedy in two ways: most of the people who think they would be good at it are wrong, and success depends on timing. With that in mind, the decisions by Theresa May last year to trigger article 50 and call a general election, in that order, look like a bad joke.

Instead of choosing a destination and organising a strategy to get there, the prime minister went on a clown-car diversion, jettisoning her parliamentary majority and incinerating her reputation as a dependable leader. She didn’t mean to do it, of course. She pursued what looked like political wisdom at the time. And it now hardly matters how the cards might have been played better. May’s most precious commodity was time, and she misspent it.

Yet time is still being frittered away. The government has postponed parliamentary debate on a bill to set the legislative framework for post-Brexit trade. It may not now come before the House of Commons for another two months. The reason is that MPs wanted to customise the law to enshrine contradictory Brexit preferences. The hard brigade want to expunge clauses that they see as gateways to retention of the EU’s customs union (or its restoration under another name). The soft squad would pass amendments to preserve such a union. Labour appears to be shifting towards that preference too.

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These are matters that a cabinet sub-committee is supposed to have resolved at Chequers on Thursday. Ministers are reported to have reached a common position on “managed divergence” from EU rules. But the decision to kick a foundational piece of Brexit legislation into April on the eve of the meeting doesn’t suggest confidence that any consensus will be sturdy. Even if the cabinet has indeed found some elegant solution to its various differences, there is no guarantee that backbench MPs will go along with them.

Abandoning the customs union is uniquely tricky because, alongside the general economic problem of trying to avoid trade friction on borders, there is the specific political problem of Ireland, where border friction is another level of dangerous. In December the UK and the EU agreed that the frontier should stay invisible. They did not resolve how that can be done if May continued to insist that Brexit means leaving the single market and the customs union. And she does insist.

That December deal is now being codified into a withdrawal agreement, of which a draft is due to be published next Wednesday by the European commission. Since this text aims to one day be legally enforceable it will be much less generous with the Irish fudge. The window of vagueness in which May has so far pretended that her no-customs-union policy and her no-Irish-border policy are compatible might then close. That in turn means the demise of her pretence that Brexit can satisfy the European Research Group (ERG) caucus of hard-right Tories, and ex-remainer, moderate Conservatives at the same time.

The prime minister may also then realise that her reliance on the hardline Democratic Unionist party (DUP) for a majority in parliament and her desire for a “stable, orderly Brexit” pull in opposite directions. She can have the backing of the ERG-DUP, or she can have progress towards a collaborative, constructive partnership with the EU. She can’t have both. And this is all before next month’s European council summit when a decision is supposed to be made about the terms of transitional arrangements and – only maybe – the initiation of talks to settle the UK’s future relationship with the EU. Tick, tock.

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The walls are closing in on May from two sides. The negotiating timetable in Brussels is tight and so is the parliamentary arithmetic in Westminster. May was desperate to avoid having to negotiate Brexit with MPs as well as the rest of the EU. When she became prime minister in the summer of 2016 she took the referendum result as her mandate. She claimed to speak and act on behalf of the “will of the people”.

Then, in the spring of 2017, she thought she saw a chance to convert that rhetorical mandate into legislative power. By sweeping up a vast majority, crushing Labour, the Tory leader would be able to enact whatever Brexit she saw fit. But the election she called had the opposite effect. Instead of being amplified and channelled through the voice of the prime minister, the people’s Brexit instructions came out diffused and distorted.

May’s legacy as prime minister will be recorded as the collision of those two dramatic electoral events: the one that put her in charge of Brexit and the one that robbed her of the means to do it her way. The Eurosceptic ultras brandish the 2016 result – the single word “leave” – as licence to demand whatever they want. But parliament, elected a year later, has the authority to define Brexit in other, more moderate ways. In popular cultural terms, the referendum was the bigger deal. In constitutional terms, parliament is paramount. The contest between them is nearing its endgame and May looks more like a bystander than a player.

• Rafael Behr is a Guardian columnist