A MAIN motive for the vote to leave the European Union was a desire to return powers from Brussels to Westminster. It is thus paradoxical that Brexiteers should be so upset that Parliament may force Theresa May to rethink her Brexit plans. A big test will come in a marathon session on June 12th-13th, when the Commons will vote on 15 amendments made by the Lords to the EU withdrawal bill. Mrs May wants MPs to reject the lot, shoring up her rocky negotiating position at the EU summit later this month. Yet if the opposition votes solidly against her, it will take just a dozen Tory rebels to defeat the government. For some mostly procedural issues, this may not matter. But five amendments would, if accepted, change the negotiations.

The most likely to pass calls for a government statement on steps to negotiate a customs union with the EU. This crosses one of Mrs May’s red lines. Still, demanding its rejection is a bit rich when her cabinet is so divided on its own customs plans. Some Tory advisers argue that merely requiring a statement will signify little. But stronger amendments calling explicitly for a customs union are due on the customs and trade bills this autumn.

Next is an amendment to make any new border arrangements in Ireland subject to agreement with the Irish government. Despite her opposition to the customs union amendment, Mrs May’s preferred backstop plan to avoid a hard border with Ireland envisages Britain staying in a customs union for many years. Indeed, her Brexit secretary, David Davis, is threatening to quit unless this backstop is time-limited. Brexiteers are also unhappy that Dublin may get a veto over any deal.

Third is an amendment to make a Commons vote on the eventual Brexit agreement more meaningful. If accepted, it would give Parliament, not the government, the power to decide what would happen if the Commons were to reject the deal. That would make it impossible for Mrs May to argue that a vote against the deal this autumn would inevitably mean Brexit with no deal at all.

A further amendment strikes out of the bill the date of March 29th 2019 for Brexit. This seems symbolic, since the date is already fixed by the two-year deadline for Brexit triggered when Mrs May invoked the Article 50 withdrawal process last year. But the amendment may leave open the option of extending the Article 50 deadline, which is anathema to Brexiteers.

The fifth key amendment requires the government to join the European Economic Area. That would mean staying in the EU’s single market, with Norway and other EEA members. The amendment will not pass, because Jeremy Corbyn, the Labour leader, does not support it, to the chagrin of many of his own MPs. Labour frets that EEA membership means accepting free movement of people and EU constraints on its policy objectives. Instead, Mr Corbyn has put forward his own proposal, to retain “full access” to the single market. He claims that Britain can get a better deal than Norway (Brussels and Oslo both insist this is impossible). But his plan will fail as no Tory MP will back it.

Mrs May makes much of Labour’s fuzzy position and internal divisions on Brexit. Yet her party’s are as bad, and not just over customs. A much-touted white paper has been deferred sine die. Moreover, Mr Corbyn’s amendment was never meant to pass. Instead the gradual Labour shift towards a softer Brexit than Mrs May plans is designed to justify the party’s voting down the Brexit deal this autumn. That vote should be the full performance, for which next week’s are dress rehearsals.