The Prime Minister will warn that a tie-up between Labour and the SNP at Westminster would be a “clear threat” to the future of the union.
Speaking in Glasgow, Mr Cameron will argue that despite their animosity the nationalists are on the “same side” as Ed Miliband’s party when it comes to increasing spending and taxes.
The SNP voted the same way as Labour 91.5% of the time in the last parliament, he will say.
“We meet here in Scotland in the middle of a massive political fight,” the PM is to say.
“We’ve got Labour and the SNP on opposite sides - slugging it out - but if you take a step back they’re really on the same side.
“You have a weak Labour party, who want more spending, more borrowing, more debt and more taxes.
“And the people who will prop them up, the SNP - who want even more spending, more borrowing, more debt and more taxes.
“Together, they pose a clear threat to the future of our United Kingdom. A coalition of chaos.
“The SNP acting as the chain to Labour’s wrecking ball, running right through our economic recovery - and it will be you who pays the price.
“With job losses, massive tax rises and an economy back on the brink of bankruptcy. In short it won’t work for Scotland, but it will hurt for Scotland.
“There’s only one party left holding the mantle of a strong United Kingdom and a strong economy; only one party speaking above these warring tribes and about what actually matters to working people: and it’s us, the Conservative Party.
“And I tell you, we will never do any sort of deal with any party that wants to break up our United Kingdom.”
The Tory leader will also pledge extra support for armed forces personnel and their families, hailing them as an embodiment of everything the union stands for.
Shadow Scottish Secretary Margaret Curran said the real threat was from the Tories.
“The biggest risk to Scotland comes from five more years of David Cameron and George Osborne - a Tory decade of austerity for people across the country,” she said.
“It’s a clear choice at this election - a Tory decade of austerity or change and a better plan with the Labour Party. Every vote for the SNP makes it easier for David Cameron to slip back into Downing Street.”
SNP Deputy Leader Stewart Hosie said: “It’s getting clearer by the day that the idea of a strong team of SNP MPs holding real power for Scotland at Westminster - and helping deliver progressive politics across the UK - has David Cameron shaking in his boots.
“More anti-Tory MPs than Tory MPs in the House of Commons - with the SNP holding the balance of power - can make sure that he is locked out of Downing Street, and put an end to the austerity agenda which is hurting hard working and vulnerable people north and south of the border.”
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