Conservative tactics of talking up the Scottish National party have been denounced as "short-term and dangerous" by Michael Forsyth, the former Tory Scottish secretary.
Lord Forsyth says senior Conservatives have bolstered the SNP in an attempt to make the case that Ed Miliband would depend on Scottish Nationalist support to enter Number 10.
Sir John Major, the former prime minister, will on Tuesday become the latest senior Conservative to warn of a Labour-SNP deal, claiming Mr Miliband would be left open to "a daily dose of political blackmail".
But Lord Forsyth said: "We've had the dilemma for Conservatives, which is they want to be the largest party at Westminster and therefore some see the fact that the nationalists are going to take seats in Scotland will be helpful. But that is a short-term and dangerous view which threatens the integrity of our country."
He highlighted the fact that George Osborne, chancellor, was quick to praise the performance of Nicola Sturgeon, the SNP leader, in the "spin room" after the first UK-wide television election debate.
"We've had George Osborne praising her saying what a marvellous performance. What she is asking for, which is fiscal autonomy, would mean that there was a gap of £8bn in the budget."
The Conservative election strategy is increasingly focused on the perceived threat to the rest of the UK if the SNP was calling the shots in a Labour government led by Ed Miliband.
With the SNP predicted to win 40 or more seats on May 7, Mr Miliband may be dependent on informal support on a vote-by-vote basis from Ms Sturgeon's party if he were to take office.
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>The idea of Mr Miliband being in the pocket of Ms Sturgeon or Alex Salmond - who hopes to be elected as a Westminster MP for the seat of Gordon - has been promoted in a series of Conservative posters.Tory candidates report that voters in England are spontaneously raising the issue of the SNP on the doorstep and the party will try to keep the issue going on Tuesday through Sir John Major's intervention.
Making a rare political intervention he will say: "The SNP's driving ambition is an independent Scotland and, as the price for their support, they will demand policies that favour Scotland at the expense, quite literally, of the rest of the UK.
"If Labour did say 'No', the SNP could withdraw support and bring down the government at any time. This is a recipe for mayhem."
Sir John is deployed sparingly by the Conservative party in the hope that his comments - in line with much recent Tory criticism of the SNP - will carry some additional weight, given his status as a former prime minister.
Jim Murphy, leader of Labour in Scotland, accused David Cameron and Sir John of playing a "tawdry and desperate" game with the electorate.
"They are a duet of cheerleaders for the SNP," he said, speaking on a soap box in Glasgow City Centre.
"We have not one but two Tory prime ministers not talking about their party but talking up the SNP. They need someone else to harm the Labour party for them."
Labour is braced to lose most of its Glasgow seats to the SNP.
Mr Murphy urged voters to return to his party, arguing that the impending vote was not a "re-run" of September's referendum on independence.
"This is about who you want to run the whole United Kingdom," he said above the shouts of nationalist hecklers. "That is either going to be Labour or the Tories, Ed Miliband or David Cameron."
Additional reporting by Jim Pickard
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