Yesterday on the BBC News programme Dateline London, the Daily Telegraph columnist Janet Daley described the Scottish National Party as an "external threat".

In itself, the moment, like the casually-delivered insult, was of no great significance, but it was symptomatic. Those who profess to care most for the Union have a strange, paranoid way of showing it.

Whether David Cameron is truly among their number remains to be seen. What is beyond doubt is that during the general election campaign the Prime Minister was content to define the SNP as that "external threat" to Westminster, to rouse English nationalism - in fact, chauvinism - against Scotland's nationalist party, and to treat a democratic choice as beyond the pale.

Where to begin? Perhaps by asking if Cameron paused once to wonder how his cheap rhetoric would affect the prospects of Scottish Conservatives. That's a small matter, perhaps, but a useful yardstick. Then you could inquire as to what remains, after this, of all those appeals to affection and unity we heard during the referendum. In essence, the Prime Minister was telling Scots that if push came to shove their MPs could be vilified, disqualified and dismissed.

That's not even the half of it. Enmity towards "the English" has no place in the language of the SNP. Those who share Ms Daley's weird world-view might dearly wish it otherwise, but both Yes campaigners and the Nationalist party have gone to great lengths to explain reality to those who are ignorant or mischievous. "Blood and soil nationalism" - another little gem from the Telegraph journalist - forms no part of the thinking of ordinary Scots. Yet members of the right-wing press, no doubt keen to defend Cameron's behaviour, have been peddling the slur relentlessly.

It's a dangerous game. First, it is a deliberate attempt to foment trouble where none has existed. What responsible Prime Minister of Great Britain and Northern Ireland would do that? Not one who means what he says when he otherwise hails the benefits of Union. One way or another, the Prime Minister has been dishonest with Scottish and English voters alike. Voters in Scotland who already treated Westminster with extreme scepticism (let's say) have been shown its ugliest face.

Secondly, there is, to be blunt, the question of stupidity. The arrival of 56 SNP MPs at Westminster ought to be proof enough that the political landscape has changed utterly. As Nicola Sturgeon has already told Cameron, Scotland's voice will henceforth be heard. Powers for Holyrood aside, there will have to be serious talking about the future structure of the United Kingdom and relationships within it if the UK is to have any future at all. Even Boris Johnson, of all people, has grasped that much. Yet Cameron has put everything at risk with tawdry provocations in a reckless attempt to stir up resentment.

The truly dangerous possibility, for honest Unionists, is that the Prime Minister doesn't care. His unexpected Commons majority has the benefit of a single Scottish MP and precious few Scottish votes. The avalanche of support for the SNP might have led him to believe that the Union will be gone sooner or later, that English nationalism - a handy counter to Ukip, after all - is the Tory future, and that "fear" of the Scots is an honest representation of English opinion. If any of that is true, Scottish Unionists might as well give up the fight now.

After all, they have a chilling truth with which to contend. In some parts of England, Cameron's loose talk certainly seems to have cost Labour votes. Ed Miliband endured a catastrophe thanks to no one but himself: that's evident from the results. But a disturbing number of English voters seem to have not a clue about how the Union and Westminster are supposed to work. The idea of a partnership in which all MPs, voters and nations are equal - the proposition defended by No voters last September - is lost on a remarkable number of people in English constituencies. They too believed there was an "external threat" to "their" parliament and, somehow, their country.

That's quite a verdict on three long centuries of Union. It also contains a verdict on Cameron. Had he been less of a hypocrite he would have been hard at work explaining the nature and virtues of the UK to English voters. Instead, he chose to alarm them. Sadly, we have not heard the end of this.