I’m not at all surprised that David Cameron has been looking somewhat dispirited during this election campaign. It must be clear to him already that all his short-term strategic calculations have gone completely awry. His party is again likely to win most seats. But, having hugged the naively willing Lib Dems tight enough to throttle them, the Conservatives are no longer likely to have a viable partner for a majority-holding coalition.

Worse, even without any of the deals between Labour and the SNP that the Conservatives warn of so hysterically, a Conservative-Lib Dem minority government would still face an opposition larger than itself, singing from largely similar song sheets. Such a government just couldn’t function. Last time, Cameron may have snatched victory from the jaws of coming first. This time, there’s a good chance that even this won’t even be a viable option.

If the polls prove accurate, then a stable government can only be bolted together by Labour, with reliance, formal or informal, on the SNP. The Lib Dems, being so keen on branding themselves as the indefatigable agents of stable rule, will no doubt hate to be left out, so they’ll probably end up in the mix. The alternative? Back to the polling booth.

Scotland has most certainly sent a giant curveball down Westminster way, and it isn’t crashing just into Labour. Cameron believed that by denying Scotland a “devo-max” option in the independence referendum, he’d get a decisive No. Having scraped a narrow No, he immediately set about appeasing English nationalists with the promise of English votes for English legislation. That was a profound mistake.

Never mind that such a thing is practically impossible, because a majority party in the UK could well find itself a minority party in England, able to pass UK legislation but unable to pass England-only legislation. Never mind that in the union there’s no such thing as England-only legislation, because the money financing it is supplied by the UK as a whole. Never mind? These are insurmountable difficulties.

If England wants England-only votes, then it’s going to have to have an England-only electoral mandate. At the moment, only Scotland appears to understand this. Fiddling about within a UK system Cameron says he wants to keep just isn’t the solution. Cameron’s narrow idea was to strengthen his support in England while damaging Labour in Scotland. Now, he says the current situation, which he engineered himself with his misguided opportunism, is something to be mortally feared. If an SNP landslide in Scotland is monstrous, then Cameron is its most recent Frankenstein. How did it come to this? The simple answer is: slowly but surely.

Democracy, at its heart, is simply rule by the consent of the people. In the UK, whoever is in power, we consent to be ruled by a government and an opposition, using an electoral system designed to encourage domination by “the two main parties”. But the Conservatives lost their Scottish mandate long ago, and has not been one of “the two main parties” in that country for decades. This should have been cause for concern for “the two main parties”. Instead, for years, they have sought merely to exploit it for themselves.

The democratic deficit in Scotland has been long-standing, and Labour believed that devolution would be answer enough to the problem. Labour believed, indeed, that the removal of the Tories from the Scottish scene could only be to their advantage. They just refused to see that Scotland’s right to be ruled by consent at Westminster was more important than any party’s narrow benefit.

Likewise, the Tories were happy to assist in the rise of the SNP in Holyrood. They knew they were done in Scotland and that therefore so was Scottish consent to two-party politics in Westminster. But instead of addressing that huge and fundamental problem, they concentrated on making mischief against Labour when the opportunity presented itself instead.

During the referendum on Scottish independence, Cameron hugged Labour tight enough to strangle it in Scotland too. Both parties believed that an in-out referendum would be enough to stop the Scottish political insurrection in its tracks. As for the Lib Dems, they simply seemed to forget that in Scotland, they were the second party.

At the last election, almost a fifth of Scotland’s Westminster seats were held by Lib Dems, who were voted for in the expectation that they would challenge the duopoly in Westminster. (No doubt Scotland fondly imagined that there might be more of an appetite for this in England than an overall 57 seats would suggest.) But instead, the Lib Dems went enthusiastically into partnership with the “main party” they’d eclipsed, their ideas about challenging the “old politics” firmly ensconced in Whitehall’s long-stay executive car park. This time, they seem unlikely to have more than one or two seats left.

Ed Miliband: I won't have Labour government if it means deals with SNP Read more

The truth is that all three parties have watched as the ability of Westminster to rule Scotland by consent has become more and more weak. They have all had their chances to engage with the problem. They have all preferred to ignore it. I’m not a nationalist by temperament. In an ideal world, I wouldn’t want the union to end. But Westminster’s intractability has exasperated many Scots, including myself, who believe that things cannot go on as they are.

I’m in no position to say how many Scots are voting SNP because they truly want independence, and how many are voting SNP because they believe that there’s no prospect of change in voting for the other parties. If I voted in Scotland, I’d be voting SNP for the latter reason – as a gamble, in the hope that it’s a way to shake Westminster out of its complacent, deluded Manichaeism.

The SNP doesn’t have all the answers by any means, any more than any one party ever does. But at least it has some questions. At least it wants dialogue with the people it seeks to represent. The Westminster parties may believe they can shut out the SNP, but that view suits the SNP just fine. Scotland has become a catalyst for change, precisely because Westminster remains so stubbornly reluctant to evolve.