Lynton Crosby, the Tories’ hard-headed Australian campaign director, is taking great pride in what he regards as one of his finest personal achievements of this year’s general election.

“Lynton loves his shoals of Salmonds and Sturgeons,” one strategist said of the Conservative party activists who pop up at Labour rallies across Britain wearing face masks of the Scottish first minister, Nicola Sturgeon, and her predecessor Alex Salmond.

Tories playing dangerous game in Scotland, says Conservative peer Read more

The “shoals” are Crosby’s lighthearted wheeze to drive home the deadly serious point that lies at the heart of the Tory election campaign – that Labour is threatening the future of the UK by drawing up secret plans to install Ed Miliband in Downing Street with the help of the Scottish National party. David Cameron intensified his warnings on Monday, which are designed to win over anxious voters in England, by talking of the frightening prospect of a “match made in hell”.

While the Crosby shoals are causing whoops of delight at Conservative HQ, party grandees and constitutional experts are concerned that the Tories are playing a dangerous game that threatens the future fabric of the UK. Some say undermining the largest pro-union party in Scotland as Labour fights for its life, while building up the SNP, is playing with fire.

Polls suggest Labour is on course to hold on to just a handful of its 41 seats in Scotland as the SNP wins up to 50 of the 59 seats available.

Lord Forsyth of Drumlean, the former Scottish secretary, has become the most senior Conservative to voice his doubts , saying that building up the SNP as a way to damage Labour was a “short-term and dangerous” game.

Vernon Bogdanor, Cameron’s former tutor at Oxford University and now professor of government at King’s College London, accused the Tories of adopting dangerous tactics after the party pledged in its manifesto to give English MPs a veto over English-only laws. He said: “It is dangerous to polarise England against Scotland and I think the whole English votes for English laws has done that.”

Bogdanor said the prime minister’s attempts to demonise SNP MPs at Westminster has echoes of Tory tactics over Irish home rule in the early 20th century, when the future Conservative prime minister Andrew Bonar Law encouraged insurrection in Ulster.

“If you are a unionist and you believe Scotland should remain in the UK, you can’t then say that the Scottish MPs at Westminster are somehow illegitimate,” he said. “That was what the Conservatives did with the Irish nationalists before 1914. Obviously their vote counts as much as anyone else’s.”

He said he has some sympathy for Miliband, who ruled out a coalition but declined to dismiss a less formal arrangement for fear of appearing to disenfranchise Scottish voters.

Bogdanor said: “You can’t say the SNP is a pariah, we won’t talk to them if they have been elected by the Scots. It is a perfectly constitutional, legitimate party. If their claims are practical claims then it is perfectly reasonable to discuss with them. You may have to do that to get any effective government at all.”

Lord Strathclyde, the former Conservative leader of the House of Lords who chaired a party commission to examine options for further devolution in the wake of the Scottish independence referendum, raised an eyebrow. He said: “Let us not worry about this too much. The SNP is a far more destructive force to Scotland than this [Tory] campaign is. Of course, the Conservative party should not play fast and loose with the union, but I am sure it isn’t. The prime minister does not want to be the last one of the union and this matters.”

One grandee, who has been at the heart of Tory Scottish politics since the 1970s, was less diplomatic. He said of the leadership’s attacks on Labour in Scotland: “That is not a good tactic in Scotland, but it might be a very good tactic in England. If the fear of Labour and the SNP is going to frighten potential English Tory defectors to Ukip to stay with the Conservatives then it is working. If I were David Cameron I would say I haven’t got much to lose because he’s only got one seat in Scotland. It is not very nice for us Conservatives in Scotland ,but it might be good politics.”

The Crosby shoals are no accident. They appeared after months of careful calculations among senior Tory cabinet ministers about the opportunities offered to the party after Scotland rejected independence. The Guardian understands Tory cabinet ministers agreed shortly after the vote last September that there could be what was described as a commonality of purpose between the SNP and the Conservatives.

Under one scenario, the two parties would agree on the complete devolution of income tax to Holyrood, paving the way for a realignment in Scotland that would see the Tories become the main centre-right party against the SNP as the main centre-left party.

The cross-party Smith commission, established after the referendum, proposed devolving a large proportion of, though not all, income tax. The prime minister also ruled out the complete devolution of income tax, known as full fiscal autonomy, as the Tories came under fire for divisive tactics in Scotland last week.

One cabinet source spoke of the depth of thinking in the party at the time. They said: “The Conservative party has been on the wrong side of all devolution questions for over 100 years. We are going to avoid that mistake this time.”

The calculations among cabinet ministers in London are alarming Tory grandees, who believe the time has come for a major constitutional convention to examine the future of the UK. Forsyth said: “I think we need to find ways of binding the UK together, of binding that partnership together.

“I think that would have been better to have been put in place before the election rather than after it. But I think you have to be very careful about an argument that says we can’t have the tail wagging the dog. It’s not a million miles away from the SNP’s argument, which is that we don’t get the governments we vote for.”