Theresa May must fail. Her first two attempts to get her Brexit deal through the Commons deserved support. They forestalled “crashing out” of the EU with no deal next month – and they took the referendum process forward.

She failed twice to persuade the Commons of this. As a result, last week her “least worst Brexit” strategy lost legitimacy. When the Commons voted to debate other options, she voted it down. She insisted she put her deal to a third vote.

In the extraordinary event of her whips persuading 75 MPs to switch sides in her favour tomorrow, the prime minister may just squeak through. But if so, it will be thanks to dozens of “nose-holders” and hard-Brexit dissidents. She will have to take her package of red lines and vague promises into a turbulent Brexit transition period, and with a bitterly divided parliament at her back. That will not be in the nation’s interest.

In the more likely event of failure tomorrow, May has promised the Commons a two-week debate on alternative forms of Brexit. Crashing out without a deal has been rejected. An alternative to May’s deal, so-called soft Brexit, would then be on the table. The option of remaining in a customs union and maintaining close ties to the single market has the support of the Labour party, the minority parties and possibly a majority even of Tories. Since May originally voted to remain, we might assume she favours it too. Only her desperate attempt to push her deal through with Tory and DUP support has kept this option from being considered.

It is utterly inexcusable that a softer Brexit in the form of a customs union has not been subject to debate in the House of Commons. It is a condemnation of parliament’s absurd polarisation and archaic submission to the government of the day. For Britain to proceed towards departing the EU without even voting on what was promised – a “frictionless” leave – would beggar belief. Now, at least, that option will be up for discussion.

But only if May loses tomorrow. Voting against her will give parliament and the country time to discuss and indicate support for a sensible compromise. It would honour the outcome of the 2016 referendum. It would enable the UK to depart the EU in good order within two to three months – with borders open, farmers happy, trade steady, Northern Ireland content and, most important, a bipartisan Commons majority behind the next stage in Brexit talks.

But first, May must lose.

• Simon Jenkins is a Guardian columnist