Britain is fast approaching a democratic showdown. The Brexit saga may yet prove to be among the worst of the current worldwide horde of nationalist populisms, or it could offer the best example of a democracy fighting back. To seize the chance of the latter will require exceptional wisdom from voters who want Britain to be true to itself.

The claim that British populism could be among the worst may seem like another example of Britons arrogantly overestimating their own importance. Worse than Viktor Orbán’s demolition of democracy in Hungary? Worse than what Donald Trump can do to international order? Yet no other populism is likely to dismantle the very country it claims to be saving. The end of the United Kingdom is a probable outcome of the hardline Brexit towards which Boris Johnson is steering the country like a demented racing driver. Brexit would also very significantly weaken both the European Union and the transatlantic alliance.

To win an election will require unprecedented cooperation between opposition parties, constituency by constituency

To be sure, measured by the standards of Hungary, let alone Turkey or Russia, British populism does not look so bad; but for one of the world’s oldest, most stable parliamentary democracies, what has happened in Westminster is shocking. The Conservative party, a centre-right broad church for at least a century, has become the Revolutionary Conservative party. Last week it purged 21 MPs, including Winston Churchill’s grandson and two former chancellors of the exchequer, all of whom are more entitled to call themselves Conservatives than Johnson. Resigning from the cabinet in disgust, Amber Rudd called this an “assault on decency and democracy”.

The conservative revolutionaries have forced through the longest prorogation of parliament since 1930, transparently lying about its purpose. “The people will never forgive the remain plotters if they don’t back down,” Jacob Rees-Mogg thundered in the Mail on Sunday, ignoring the fact that the last time the people spoke through the ballot box, in the 2017 election, they gave us precisely this parliament of “remain plotters”.

And this is where the good news begins. Many people around the world have been laughing at the House of Commons, with its antiquated procedures and theatrical Speaker. Actually, the Westminster parliament is doing us Britons proud. Over the last couple of years, those green leather benches have seen great speeches, deep emotion and courage, with members putting the national interest before personal and party advantage. Now parliament has stopped the populist bullies in their tracks, swiftly passing a law that obliges the government to ask for an article 50 extension if no deal has been agreed with the EU and approved by MPs by 19 October. Were Johnson to refuse to do so, as he is currently threatening, then he would have broken the law and could, ultimately, be sent to prison.

What should Britain’s democratic showdown look like? Former prime minister Tony Blair says we should go straight to a second referendum. That is exactly where we should end up, but there are not the numbers in this parliament to vote through the necessary legislation. It is just possible that Johnson will abandon all his red lines and scramble towards a slightly modified version of the deal negotiated by his predecessor, Theresa May, which could be agreed at the EU summit on 17-18 October and may even scrape through this parliament.

Facebook Twitter Pinterest Boris Johnson speaking in Dublin on Monday. Photograph: Lorraine O’Sullivan/AFP/Getty Images

Absent that unlikely turn, the next step is a general election. Since Johnson cannot be trusted, the opposition parties have agreed to wait until an article 50 extension is secured before consenting to that election. If Johnson resigns rather than request an extension, that will get us to an election by a different route. Much as their patience has been tried, the EU27 would be wise to signal a willingness to grant an extension of at least two months, on the understanding that Britain’s sovereign parliament will have brought about a general election within that time. The alternative, a no-deal Brexit, is a lot worse for both sides of the Channel and future relations across it.

In this election, even if it is held after Johnson’s “do or die” 31 October deadline has passed, the advantage will be with the hard Brexiteers. They have a single clear objective – get Britain out of the EU – and their vote is only divided two ways, between the Conservative party and Nigel Farage’s Brexit party. They could make an electoral pact. Farage claims “an alliance between Boris and myself” would be “unstoppable”.

The other side does not have a single clear objective. Many, myself included, are for holding a second referendum, but others are just for a softer Brexit. And our vote is potentially split seven ways – between Labour, the Liberal Democrats, the Greens, the Scottish National party in Scotland, Plaid Cymru in Wales, the Independent Group for Change, and the now quite numerous former Tory MPs, some of whom may stand as independent Conservatives.

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To win this election will therefore require unprecedented cooperation between opposition parties, constituency by constituency. Labour and Lib Dems, above all, should agree to step back wherever the other has a better-placed pro-referendum candidate. It will also require skill on the part of voters, identifying the candidate to back in each constituency.

Social media and youth turnout will be pivotal. Officials in Downing Street have told Katie Perrior, a former No 10 communications director, that one reason they want an early election is to pre-empt incoming students registering to vote and potentially swinging the result in their university towns. I trust that indicates to all students exactly what they need to do.

Yet even if the opposition parties and independents get a majority, they still need to keep it together to legislate for a second referendum. Even if we reach a second referendum, we still need to win it. Even if we win it, we will still have the huge task of showing those who in 2016 voted for Brexit, often for economic or cultural reasons little related to the EU, that we have heard them loud and clear. But at least there is still a chance – perhaps a last chance – for one of the world’s most venerable democracies to help turn the global tide against nationalist populism.

• Timothy Garton Ash is a Guardian columnist