Economist David McWilliams caused some excitement last week when he set out a scenario for a united Ireland, in which the UK votes to leave the EU, causing Scotland to leave the UK and unionists to lose financial faith and cultural affinity with the Union.

This is nothing republican enthusiasts have not been saying amongst themselves but it was nice for them to hear it from an objective source. Demand for such speculation can only increase as the prospect of out-voting or out-breeding unionists appears to recede in the medium term – no doubt an economist could plot it on a graph. But there is a far more direct threat to Northern Ireland’s link with Britain than McWilliams’s long chain of possibilities. However the vote goes in next month’s European referendum, the Conservative Party looks set for civil war, clearing a path to Downing Street for Labour leader Jeremy Corbyn and his ambitious shadow chancellor John McDonnell.

This prospect is considered completely implausible by political observers, ironically in part because Labour has lost Scotland to the nationalists. England preferred David Cameron to Corbyn’s predecessor Ed Milliband so would certainly not favour an even more esoteric north London leftist.

Sectarian

Corbyn polls heroically badly as a potential prime minister but support for his party is holding up well, at 32 per cent compared to 34 per cent for the Tories. It seems that in political terms, English Labour voters are sectarian – they will back their own side no matter what. Northern Ireland is only the UK’s second most electorally predictable region, after the Labour heartland of the northeast of England.

Corbyn or McDonnell might have no hope against Cameron but he will not be their opponent at the next scheduled general election, by when he has vowed to stand down. If the referendum triggers an earlier ballot, there is no guarantee that Cameron will be Labour’s opponent – both factions of his party are already calling for his head. All his suggested successors are ‘divisive’, as we must now call any conservative who is not carefully bland – and all would have risen amid utter chaos in their party.

Labour still could not win, the pollsters assure us. But for unionists in Northern Ireland the most dangerous outcome is a narrow Labour defeat. Then Corbyn or McDonnell could be put in office by the Scottish nationalists, attracted by the very prospect of breaking up the UK. If defeat is not narrow either man might try again, as their tendency has enough support among Labour members to endure, while McDonnell has the guile to rebrand himself should Corbyn need to be dispatched.

Since assuming their posts, Labour’s leader and shadow chancellor have been at pains to point out that they support and have always supported the peace process. This is somewhat at variance, to put it mildly, with their decades-long support for Sinn Féin and equivocation over IRA violence, extending to after the ceasefires and the Belfast Agreement. Corbyn is on record endlessly opposing the consent principle and in 1998 McDonnell told An Phoblacht that he rejected a settlement short of a united Ireland.

Formula

Should Corbyn or McDonnell reach Number 10, ditching Northern Ireland would be as simple as turning off the money. The UK’s regional funding formula, on which Belfast’s £10 billion subvention depends, is merely a procedural convention that can be changed at the stroke of a pen – a pen the SNP would immediately brandish. Because of the odd way benefits are half-devolved, Stormont could be impoverished without causing much actual poverty. The consent principle does not technically require subsidising consent. This might be among Corbyn or McDonnell’s most popular policies in Britain.

Unionists seem remarkably blasé about the risk, however slight, of a British cabinet that views them as illegitimate colonists who should be abandoned to their fate. To some extent they are inured to an attitude they suspect lurks eternally in London. They are also oblivious to unpopularity, as the pathetic need to be liked is an Irish characteristic.

But this may be blinding unionists to how deeply Corbyn and McDonnell’s lifelong worldview is tied up with breaking the Union. If either man ever gets in, Northern Ireland is on the way out – whatever happens between Britain and Europe.