If the Conservatives win a majority on 12 December, as they are favourites to do, they will claim a mandate to “get Brexit done”. As a result, there is an extremely real possibility that, by the time of the next scheduled election in 2024, the United Kingdom as we know it will no longer exist. Scotland may by then have voted to become an independent country. Northern Ireland may have voted to unify with the Irish republic. But you would hardly know any of this from the general election campaign so far.

In the leaders’ debate between Boris Johnson and Jeremy Corbyn, there was much discussion of Brexit. But there was no discussion about Brexit’s consequences for the parts of the UK – Scotland and Northern Ireland – that did not vote for it. Nor was there a single word about Brexit’s effect on the unresolved divides in Ireland. This was genuinely remarkable. For the past three years, the issue of Ireland has been at the very core of the argument about Brexit. But now, from the leaders of Britain’s two main parties, there was absolutely nothing. Not for the first time in British political debate, it was as though Ireland simply did not exist.

There is a black hole not just in the election campaign, but in our politics where a unified nation is supposed to be

Yet this conspiracy of inattention ignores two looming existential challenges to the cohesion of the country. The possibility that Scotland and Northern Ireland might cease to be part of a union with England and Wales after Brexit does not emerge from a cloudless sky. The Scottish nationalist government has been clear since 2017 that it is demanding an independence referendum because it wants Scotland exempted from Brexit. On Wednesday , the Scottish National party reiterated its demand for this to happen in 2020. Northern Ireland’s unionists are now in revolt against a Brexit deal that would cut them off from Britain. All this has been in the news for months. Yet it forms no part of the mainstream campaign debate.

In last Friday’s studio quizzing of the four main party leaders, , in which Johnson and Corbyn were joined by Jo Swinson and Nicola Sturgeon, things improved a little. There was still not a single word about Ireland. But the presence of Sturgeon meant at least that Scotland formed part of the debate. Significantly, Corbyn announced that he would not allow the SNP to hold a second independence referendum in at least the first two years of a Labour government. Sturgeon responded confidently that she did not expect Corbyn to stick to that pledge if Labour found itself needing SNP votes in order to govern after 12 December. Corbyn’s own adviser Lord Kerslake said something similar this week.

Perhaps things will improve in the remaining two weeks of the campaign. But don’t hold your breath, especially not on a level that remotely measures up to the seriousness of what may be so imminently at stake. In part, that is because the “national” campaigns of the three main parties are aimed overwhelmingly at English voters. Since England provides 533 out of the 650 MPs in the House of Commons (82% of the total), this is not surprising. But it means that even the SNP manifesto launch is treated as essentially a Scottish sideshow. As so often, much of England treats the rest of Britain with what the historian Ali Ansari somewhat generously calls “a calculated and studied air of neglect”. Right now, however, that habit of neglect is potentially lethal.

The Liberal Democrat manifesto is the only one to discuss the UK constitution, but the party ‘is not taken sufficiently seriously for its approach to have any possibility of cutting through’. Photograph: Hannah McKay/Reuters

The newly published manifestos of each of the three main all-UK parties help to explain why this is so. But in each case the contribution they make to the poisonous overall effect is different. The Tories are defiant about it. Labour hopes to wish it away. And the Liberal Democrats are not taken seriously. The net effect is to pour petrol on the fires, not to douse them.

The Conservative manifesto contains plenty of rhetoric about the UK. It claims that “the United Kingdom is the most successful political and economic union in history” and it says that Conservatives “stand for a proud, confident, inclusive and modern unionism that affords equal respect to all traditions and parts of the community”. But this rhetoric flies in the face of YouGov polling in June that showed 63% of Tory party members were willing to accept Scottish independence and 59% a united Ireland if Brexit could go ahead.

It is also at odds with the contemptuous way that the Tory party under both Theresa May and Johnson has actually handled Scottish and Northern Irish responses to Brexit. That reality was provocatively on show again this week when, with all the absolutist confidence of a lordly Bourbon, Johnson dismissed any possibility of negotiation with the Scots over a second independence vote. Like May before him, Johnson appears indifferent to – and may even welcome – both the anger that his Brexit deal provokes and the formidable nature of the forces ranged against him, in particular of the SNP.

Labour’s problem is altogether different. Its manifesto has far less to say about the union. Indeed, remarkably in some respects, the words “United Kingdom” do not occur anywhere in its 107 pages. Yet Labour definitely has an ambitious unionist policy. It says it is opposed to Scottish independence, which it says would be “economically devastating”. What Scotland needs, Labour says, is not independence but money. Labour promises Scotland “transformative investment” worth “at least around £100bn”.

This is classic centralist social democratic unionism of the postwar kind. The Labour Scottish secretaries of earlier eras, people like Willie Ross, Bruce Millan and George Robertson, would have known it for what it is. But Corbyn cannot call it by its name. The anti-imperialist campaigner is in deep denial about the fact that he leads a unionist party pursuing a unionist policy. Many in his party take a similarly ambivalent view about the breakup of the UK.The Liberal Democrats’ manifesto embodies another problem. It is the only one that has detailed new things to say about the way the United Kingdom could work better constitutionally. Swinson’s party supports “home rule for each of the nations of a strong, federal and united United Kingdom”. It mistakenly stops short of advocating an English parliament, but it is open to devolved parliaments in regions that want them, such as potentially Cornwall or Yorkshire. Yet the difficulty with the Lib Dem proposals is simply that the party is not taken sufficiently seriously for its approach to have any possibility of cutting through.

The net result is a black hole not just in the election campaign, but in our politics where a unified nation is supposed to be. Brexit has accentuated threats to the UK – along with many other wider problems in British politics – that were already gathering well before 2016. Perhaps, as the historian David Reynolds has recently argued, these are rooted in the country’s long postwar complacency about its institutions. Be that as it may, the opponents of those institutions are now more articulate and more focused than those who want to reform and defend them with credibility. If none of the main parties can speak effectively for the national union in ways to which majorities in all the component nations can respond, then who else can? For without believable voices to speak for it, the union, as we may be about to discover, could soon be past saving.

• Martin Kettle is a Guardian columnist