GETTY The end of Britain as we know it? The fight for Scottish independence isn’t over.

In Scotland, a rebellion is brewing the likes of which Britain has not experienced in nearly a century and that, if unchecked, will change the United Kingdom for good.

The Scottish National Party (SNP) lost a referendum on Scottish independence last September; now, just seven months later, they are set to win an extraordinary, historic election victory in the British elections, which take place next month. It wasn’t supposed to be like this. The question is no longer whether they will win the Scottish portion of the election but by how much. The SNP lost the war but they’re winning the peace.

The latest opinion polls suggest that, however improbably, the nationalists could take as many as 50 of Scotland’s 59 constituencies. They have never previously won more than 11; five years ago they took just six. Even if the SNP vote is squeezed in the final weeks of campaigning they are still liable to be the largest party in Scotland.

To appreciate the full scale of this revolt, it is necessary to remember that in 2010 the Labour party, dominant in Scotland for more than half a century, won twice as many votes as the SNP. Labour won more than 50 percent of the vote in no fewer than 18 constituencies in its west of Scotland heartlands. This is the kind of political fortress that is supposed to be impregnable. Now, however, Labour’s defenses have been breached.

For the first time ever the SNP will be a relevant political force at Westminster. And since the Liberal Democrats seem likely to lose half their seats it is possible that the nationalists will become the third largest party in the House of Commons. The SNP’s rise is, in many ways, the greatest domestic threat to the integrity of the British state since Sinn Fein won a landslide victory in southern Ireland in 1918. That laid the way to Irish independence and it is quite possible the SNP’s victories will, eventually, lead to another, this time peaceful, constitutional revolution.

Launching the SNP’s election manifesto in Edinburgh last Monday, Nicola Sturgeon, the party’s new and impressive leader, was careful to say that this election is not, per se, about independence. It is instead simply about choosing which party can best “stand for Scotland.” But the thought of another referendum is never far from anyone’s minds in Scotland these days. Independence is still the driving force that animates the SNP and, for now, the party is prepared to play a longer game.

The nationalists believe they have time—and demography—on their side. Only 45 percent of Scots voted for independence last year but the Unionist campaign relied on the votes of older voters aged more than 65 to carry the day. Their younger compatriots voted for change. Of course, demographics are not destiny, but as matters stand, each year the population profile shifts a little towards the SNP.

In any case, in Scotland, this election is merely the first half of the game. Next year’s elections to the Scottish parliament, where the SNP have been in power for eight years, will prove just as important. Another nationalist victory then might make it hard to avoid holding another referendum on the national question. Ms. Sturgeon says there would need to be a “substantive” change in circumstances to justify another “once in a generation” referendum but the thing about politics is that circumstances are always changing. No wonder many Scots fear a so-called “Neverendum.”

The Unionist parties—Tory, Labour and Liberal Democrat alike—are being pummeled by a relentless logic. If you voted Yes to independence last September why would you vote for a Unionist party this May? The national question dwarfs all other considerations in Scotland these days. Identity has replaced class as the basis for Scottish politics and the SNP, which parades itself as “Scotland’s champion,” cannot be beaten when politics is a flag-based business. The SNP is a movement just as much as it is a traditional political party. It now has more members than the British army has soldiers.

The SNP’s rise has almost certainly ended whatever faint hopes Labour once had of winning an overall majority next month. Labour most likely cannot win without Scotland. Which is one reason why David Cameron has consistently raised the “chaos” that might ensue if Labour was forced to come to some kind of post-election arrangement with the SNP.

On the one hand, Cameron is alive to the fact that 60 percent of English voters dislike the idea of a Scottish, separatist, party wielding a hefty measure of influence in London. On the other hand, he is aware that the Tories’ chances of remaining the largest party — and even of coming within a sniff of a majority — depend on Labour being hammered in Scotland. The SNP surge, in other words, benefits Cameron, who enjoys suggesting that Ms. Sturgeon would have Ed Miliband, the Labour leader, in her pocket in the event of a hung parliament.

But if it benefits Cameron in the short-term, it is also a strategy that appalls many Unionists who suspect Cameron’s tacit encouragement of the SNP risks endangering the Union in the medium-to-long term. Senior Conservatives even allege that SNP support for Labour—though not in any formal coalition—“undermines” democracy, though how it does that remains a matter of some mystery.

For its part, Labour has tried to argue that a vote for the SNP benefits the Conservatives. In previous elections this tactic has helped squeeze the SNP vote in Scotland. This time it seems as though voters don’t care.

In any case, the parliamentary arithmetic means that a vote for the SNP is most probably actually a vote for a weak Labour government. Unless Cameron can cobble together a majority with the Lib Dems and, perhaps, Northern Ireland’s Unionist parties, it’s likely that, together, the SNP and Labour could, as Sturgeon says, “lock the Tories out of Downing Street.” Labour in Scotland, however, cannot admit such a possibility even as Labour in England cannot afford to discount it.

Paradoxically, however, the SNP’s strong electoral position may not leave it in a position to influence the future governance of the UK as much as its army of supporters thinks. Having repeatedly ruled out any arrangement with the Tories, the SNP have nowhere to turn other than Labour.

Which in turn gives Labour space to reject SNP calls for higher public spending, scrapping Britain’s nuclear deterrent and, indeed, devolving further responsibilities from Westminster to the Scottish parliament in Edinburgh. They can dare the SNP to vote down a minority Labour government and, subject to certain constitutional and procedural technicalities, risk another election. It is unlikely the SNP would accept such a challenge for fear that doing so could benefit the Tories.

Then again, this is a win-win election for the SNP. The next UK government is likely to be weak and unpopular in short order. The SNP might not bring down a Labour government but nor will they offer it unqualified support. The nationalists intend to be in power but not in office. And if Labour rejects the SNP’s calls for a more populist approach? Well, the SNP will then use this as confirmation that Labour cannot be trusted to act in Scotland’s interests. (Conveniently, the national interest often seems to overlap with the SNP’s interest.) In 2016 the nationalists will run against any Westminster government even as they defend their own record in office in Edinburgh.

A Tory victory, however improbable it may seem, would be just as useful to the SNP. If Mr. Cameron wins a second term he will do so on the back of English votes and a campaign that has, increasingly, played one part of Britain off against the other. “Scaremongering” about undue Scottish influence at Westminster is not something likely to be forgotten north of the border.

Moreover, it would add weight to the SNP’s longstanding assertion that (left-wing) Scotland and (right-wing) England are such different polities it no longer makes sense for them to be part of the same nation state. If the Tories ask Scots to choose between their national identity and their political Unionism, Unionism will perish. That seems a gamble the Conservatives, notionally the party of the Union, are prepared to make.

What’s not in doubt is that the sparks lit by the referendum campaign in Scotland continue to burn. There has been no return to politics as usual. Parliamentary hustings are attracting twice the audiences they managed five years ago and, across the country, there are far more such meetings than was the case in 2010.

The referendum tapped a yearning for a bigger, better, grander kind of politics and, once unleashed, that kind of sentiment cannot disappear overnight. Far from depressing Yes voters, defeat in the referendum has redoubled their enthusiasm for change.

Britain’s first-past-the-post electoral system also benefits the SNP. Their vote is impressively united and the Unionist opposition is split in three. That means any number of constituencies previously reckoned invulnerable may yet be stormed by the SNP. There are, perhaps, no more than a handful of Labour seats that are truly “safe.” Meanwhile, the Liberal Democrats may lose as many as ten of their eleven Scottish seats and the Tories are unlikely to add more than one additional seat to their current, forlorn, total of one.

It all adds up to a rout. The most recent constituency polls suggest that in many Labour seats there has been a 25 percent swing to the SNP. Leading Labour figures, including Jim Murphy, the Labour leader in Scotland and Douglas Alexander, the likely foreign secretary in a Labour government, could yet be unseated by the SNP. As a Labour spokesman confirmed, “There is no gloss that can be put on this.”

Another Labour MP was still more succinct: “I think I’m fucked.”

Alex Massie writes for the Spectator, the Times and other publications.