On 16 February 1886, Lord Randolph Churchill confided a plan to destroy his Liberal opponents to the Conservative lawyer Gerald FitzGibbon. It was a risk, he implied. But if William Gladstone’s Liberal administration proposed home rule for Ireland, “the Orange card would be the one to play. Please God it may turn out to be the ace of trumps and not the two”.

Before tobacco and alcohol killed him – possibly with the help of tertiary syphilis (historians differ on whether he added the pox to his wages of sin) – Churchill was one of the most formidable and unscrupulous Conservative politicians of his age. The doubts in his letter – “please God it may turn out to be the ace” – suggest even he paused before appealing to Protestant sectarianism. From the point of view of the self-interest of the Tory party, Churchill’s tactic, inciting the Protestants of Ulster to reject a Catholic-dominated all-Ireland parliament, was a dazzling success.

Gladstone beseeched the Commons to be generous and remember there was a difference between “giving with freedom and dignity” and “giving under compulsion – giving with disgrace, giving with resentment dogging you at every step”. But respectable English opinion did not feel the inferior Irish could govern themselves. Imperial British nationalism, and religious and racial contempt, drove 93 Liberal MPs to vote down home rule. They left their party and formed an alliance with the Tories, which went on to govern Britain for 17 of the next 20 years.

History vindicated Gladstone, however, and damned Churchill. What wasn’t given freely was taken under compulsion. After the Easter Rising, the Irish Free State eventually left the United Kingdom and the wars in Ireland continued until the Good Friday agreement of 1998. Maybe given the sectarian tensions between Protestant and Catholic they would have happened anyway. But no one can deny that the Tories inflamed tensions that tore the old kingdom of Britain and Ireland apart.

David Cameron is as unscrupulous as Churchill, if nowhere near as formidable. He is trying to sneak an election victory by inflaming English nationalism. In the process he is inflaming Scottish nationalism. It is a truth nowhere nearly as widely acknowledged as it should be that English and Scottish nationalists are like brawling drunks leaning on each other for support. They need each other. The better the Scottish nationalists do, the worse Labour will do and the better able the Conservatives will be to cling to power. If Cameron loses, Scottish nationalism will allow his party to say that an incoming Labour government is controlled by “alien” Scots, who have no right to vote in Westminster. Likewise, if the Conservatives win, the SNP wins too. It will be able to push for independence for Scotland from the rule of “alien” Tories.

To use the only piece of sociological jargon that has enriched the language, the Tories need the Scots to be their “other” and the SNP need the Tories to perform the same useful task for them. Without the “other”, how will they whip “their” people into line and scare them into marching to the polling booths and crossing the right boxes?

The heirs of the Ulster Orangemen Churchill incited to rebel against home rule see the dangers better than half the commentators in London. Of course Scottish nationalist MPs have the right to support what government they please, said Nigel Dodds, the leader of the Democratic Unionist party in Westminster. “Glib and lazy talk about SNP MPs somehow not being as entitled to vote in every division in the Commons, as any other British MP, simply fuels nationalist paranoia.”

The DUP, you will remember, is the political wing of “Dr” Ian Paisley’s Free Presbyterian Church of Ulster. When Paisley was alive, it incited hatred against Catholics and came within a whisker of inciting violence. It remains a party of biblical literalists. A large proportion of its members have trouble with the existence of dinosaurs.

It is a measure of Cameron’s cynicism that a sect of Protestant ultra-fundamentalists are more responsible than the supposedly sensible and moderate leader of the British Conservative party.

I am not sure that all members of the English left sense the dangers of nationalism either. If the Tory press runs the absurd fantasy that the SNP will bring socialism to Westminster, I meet many equally absurd English leftists, who hope and believe the Tories are right. Unlike their French and Scottish counterparts, they have always distrusted patriotic feelings about their own country. With good cause, you might say, as borders divide humanity. The worst, however, respect everyone’s borders but their own. Like sex tourists, they search the world for countries where they can enjoy the radical fantasies they can’t get at home. After the Soviet Union, Cuba and Venezuela, we now have the English left, or at least a vocal element within it, seeing Scotland as their dreamland.

The “leftwing” Nicola Sturgeon won the endorsement of Rupert Murdoch’s Scottish Sun. I wasn’t surprised. Alex Salmond’s willingness to lobby on Murdoch’s behalf was so brazen, it became a subject for the Leveson inquiry . The Scottish Labour party has been trying without success to persuade reporters to ask whether the price for Murdoch’s continuing support is a promise from Sturgeon to order nationalist members of parliament not to support Labour’s plans to break up his media empire.

It’s a good question, and when leftists have asked it, they should ask whether they understand that the first concern of any nationalism is to erect barriers, not build social democracy. I don’t think many do. If they did, English leftwing politicians would be bellowing out their plans to stop the growth of an ugly and resentful English nationalism by giving greater power to English cities. That they are not shows they are as blind as Cameron to future dangers.

Churchill did not want to help push Ireland into a conflict that would last a century. But those who came after him had to live with it. I am sure that Cameron does not want to break up the union. He’s not good enough to think beyond the next news cycle. He merely wants to scramble through the next few days and cobble together a deal that will somehow keep him in power. But petty mediocrities can bring calamities as surely as grand villains. All that remains constant is that the rest of us must live with the consequences.