It’s crunch time. After two years of backroom deals, faux Tory rebellions, pizza plotting and dull political theatre that turns most of the country off, Theresa May is finally on the edge of agreeing a Brexit deal she can present to parliament.

Whenever it comes, we know her hands are tied. Whether she goes for a hard Brexit that gives up full access to the single market and the customs union, or instead opts for some form of customs union and no hard border in Ireland, her precarious DUP-enabled majority can’t deliver either. Between her irrevocably split backbenchers and the hardline DUP she cannot get a majority for any kind of Brexit deal. This means she needs the votes of at least some Labour MPs.

We already saw this dynamic play out in July when May successfully wooed four – Frank Field, Kate Hoey, John Mann and Graham Stringer – to scrape through her customs bills. Without them the government would have faced a major crisis with a vote of no confidence and a general election on the cards. Such an opportunity cannot be missed again. The vote on the Brexit deal in parliament will be seismic. That’s why Momentum consulted our members to get a steer on what they thought. Our findings? Ninety-two per cent of members want every single Labour MP to vote down a Tory Brexit deal if it fails the party’s six tests.

The fundamental reason is obvious. Any Tory Brexit – hard or soft – will be conducted in the interests of the few. Big business, bankers and press barons will be the beneficiaries – not the young, underpaid and precariously housed, or those who voted to take back control after seeing their communities ripped apart by decades of globalisation. For all the bureaucratic jargon, Brexit is a set of policies like any other, and a certain class interest will underpin May’s deal as it does any other Tory policy. They are the party of big business and the ruling class. Any Labour MP who gives the prime minister their vote will be siding with a reactionary Tory establishment.

They’ll also be making a huge strategic error. With schools sending parents begging letters to keep the lights on, rough sleeping going through the roof and our NHS about to enter yet another winter of waiting lists, ambulance queues and beds shortages – we cannot wait until 2022 for a change of government. The only way to achieve that is a general election – as soon as possible – so Labour members can get out campaigning, make the arguments on the doorstep and deliver us the majority we need.

The roadmap to a general election is now clear. Vote down May’s Brexit deal, the Tories implode and Labour pushes for a vote of no confidence. The alternative – propping up a Tory Brexit that benefits the establishment and throwing away the chance of a general election – will be kryptonite to Labour members across the country, who will rightly interpret a vote with May as a vote for four more years of Tory rule rather than a Corbyn-led Labour government.

Like the rest of the population, Momentum members have many different, often complex, views on Brexit. But on voting down the Tory deal, they’re in near-universal agreement. And with increasingly vibrant, active local parties and a Labour membership of more than 500,000, any Labour MP who votes with the government is going to have a hard time explaining themselves back in their constituencies.

• Becky Boumelha is a member of Momentum’s national coordinating group