For all the huffing and puffing, for all the complex games of bluff over the Irish border and the transition period, the important facts of the Brexit deal remain relatively simple. Between the red lines of the DUP, the Tory right and the Tories’ liberal wing, there ought to be no deal which can get a majority in the House of Commons. This situation should bring down the government, or stop Brexit from taking place, or both.

The only way that Theresa May can be sure of delivering a majority is with Labour assistance. The Labour leadership will not wobble in its resolve to oppose the deal – doing so would pass up a golden opportunity to bring down the government and tear the Corbyn project apart. The Labour MPs who might back the deal are almost all from the centrist wing of the party, and fall into three categories: genuine Leavers, timid MPs representing Leave seats and those who profess concern about a no-deal scenario arising from a government defeat.

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Any Labour MP who fails to vote against the Tory Brexit deal would be making the biggest mistake of their political lives. Corbyn’s opponents in Labour have bet their collective credibility on claiming to be better at parliamentary process and being anti-Brexit. Now, some of them stand on the brink of voting through a catastrophic version of Brexit, and falling for the wholly predictable propaganda line that voting down the deal would result in no deal. There is simply no excuse for believing this – if the deal falls, parliament will get to decide what happens next.

The idea of propping up the Tory government in a vote of this kind sits well with a certain tendency of the Labour right, appealing to the national interest and statesmanlike conduct. It is the same tendency that has seen establishment Labour leaderships whip with the government on issues such as welfare reform, and which was most dramatically demonstrated with the formation of the National Government by Ramsay MacDonald. In times of national crisis, there has always been a portion of the parliamentary Labour party that has been willing to put a painful sense of self-regard ahead of the petty mission of being Labour MPs.

The consequences of a Tory Brexit deal passing are serious. By propping up the government, it is likely to guarantee the continuation of austerity and welfare cutbacks, which have devastated lives and communities, for a further four years. Its substantive effect will be to give Liam Fox the power to reshape the British economy by signing regulation-busting trade deals, and will mean a huge loss of rights and protections for workers, migrants and the environment. By handing victory to the Brexiters, it will embolden the most reactionary elements in British politics and their narratives.

The past few months have witnessed the development of a new movement in Labour, with 86 % of members wanting a fresh referendum. A large majority is also in favour of holding MPs accountable in local parties. There are good reasons to defy the party whip if the leadership is acting in defiance of the democratic will of members, or if the consequences of following the whip are morally indefensible. But voting down the deal is the official policy of Labour conference, passed with 90% support. Even if you don’t want to stop Brexit, blocking the deal needs only be a means of giving space for alternatives to the Tories’ agenda.

So Labour’s grassroots – the overwhelming majority who are on the left and who want to oppose May’s deal – must be clear about what their response would be if the government passes its Brexit deal with Labour support. After Labour’s conference, the process for deselecting an MP is much easier, with only a third of local branches or local affiliates required to trigger an open contest. Any Labour MP who fails to vote down the Tory deal must not be allowed to contest the next election as a candidate.

Labour MPs have often been led on to the wrong side of history by the heat of a national crisis or the allure of the political establishment. But at almost every turn, from the National Government of the 1930s, to the neo-liberalism of New Labour and the austerity politics that followed it, those who abandon the will of the Labour movement for lazy answers inside the Westminster bubble have lived to regret it. Those now in parliament would do well to remember that.

• Michael Chessum is a freelance writer and socialist activist