The United Kingdom is holding its third national election in five years on Thursday, the fourth if you also count the 2016 Brexit referendum that permanently altered the country’s fate. Polls right now suggest Boris Johnson’s Conservatives will probably win an overall majority. But the race has narrowed, polls have been very wrong before, and Britain’s first-past-the-post election system is notoriously hard to predict. It could go either way.

The issue of Brexit has dominated this election, given the stakes. If Johnson gets his majority, he will have the support he needs to finally take the country out of the European Union next month, under the terms of the deal he negotiated with Brussels earlier this year. If there’s a hung Parliament, the country is likely headed for a second referendum on leaving the EU.

But the stakes could be even higher. The election could determine whether the United Kingdom continues to exist as a country in its current form.

One of the greatest ironies of Brexit is that for all Britain’s uneasiness with the shared sovereignty and open borders of the European Union, the United Kingdom is itself an experiment in shared sovereignty and open borders (albeit one that has not always been voluntary for all its members).  Brexit puts both experiments in danger.

The threat of leaving the EU has given new momentum to proponents of Scottish independence. Nicola Sturgeon’s Scottish Nationalist party is battling it out with Labour for swing seats in Scotland and polls suggest it is likely to regain at least some of the seats it lost in 2017. The unambiguously pro-Remain SNP has benefited from Labour’s wishy-washy Brexit stance as well as the unpopularity of leader Jeremy Corbyn. In her battle to win over Labour voters, Sturgeon has somewhat downplayed her party’s defining issue—Scottish nationalism—but the party is still calling for a new independence referendum in 2020. (Scots voted by a 10 -point margin to remain part of the U.K. in the last referendum in 2014.*) If Conservatives cannot manage to form a government, Labour will very likely need the SNP’s support if it wants to govern. Corbyn opposes a new referendum, but if he wants to be prime minister, he might have to agree to it. In the event that London refuses to permit a referendum, the SNP has not ruled out holding an unauthorized “Catalonia-style” independence vote: a pretty alarming prospect given what’s been happening in Catalonia.

Then there’s the open question of Northern Ireland’s future—which emerged as the surprise sticking point in the Brexit negotiations. EU membership has made Northern Ireland’s hard-won peace possible, allowing the region to be politically part of the United Kingdom while economically and culturally tied to the Republic of Ireland. The pro-U.K. Democratic Unionist Party, which was until recently in a coalition with the Conservatives, is likely to lose some seats, while the Irish nationalist Sinn Fein—which holds seats in Westminster but doesn’t actually participate or vote—is likely to pick some up. And a Corbyn-led government would mean that for the first time, the U.K. has a prime minster who supports Irish unification.

A poll from September asking Northern Irish voters if they would prefer to remain in the U.K. or be part of the Republic of Ireland showed a statistical tie. And that was before Johnson’s controversial Brexit deal, which would leave Northern Ireland under EU customs rules, effectively creating an economic border down the Irish Sea.

As a former adviser to Tony Blair put it, “Paradoxically, Mr. Johnson and Brexit may have done more for a united Ireland than the [Irish Republican Army] ever did.”

But there’s even more compelling evidence that there really is a threat to the future of the union: The English, and in particular English Conservatives, just aren’t that enthusiastic anymore about keeping the union together.

A recent poll showed that 53 percent of English Conservatives would still support Brexit if it led to the unraveling of the Irish peace process. Seventy-seven percent would still support it if it led to a second Scottish referendum. This means that the party that is officially called the Conservative and Unionist Party is now less unionist than the party led by an erstwhile IRA supporter who is likely to partner with Scottish nationalists.

The prospect of a U.K. breakup might still seem far-fetched, in part because national border changes are so rare these days, but British politicians on both the left or right are making it seem less fantastical.

Correction, Dec. 12, 2019: This post originally misstated that Scotland voters rejected independence by a 10 percent margin. It was a 10 percentage point margin.