The Prime Minister insisted that the Nationalists want to see Westminster and the UK and fail and would only be interested in “breaking up” the country.
It comes despite labour leader Ed Miliband insisting that he would rather lose the election than do a deal with the SNP, while Nicola Sturgeon insists that the SNP would be “responsible” at Westminster.
Mr Cameron warned Labour will lose so many seats to the SNP in Scotland that Mr Miliband won’t be able to form a majority Government on his own.
“He can only become prime minister with SNP support,” the Tory leader said.
“Nicola Sturgeon and Alex Salmond will extract a heavy price for that support, vote-by-vote in the House of Commons.
“That price is even more borrowing and taxes to pay for more welfare.
“That is the road to ruin. It would be a calamity for our country.”
He added: “The SNP only really have one interest and that is to be break up our country.
“They don’t want a Westminster government to be a success, they don’t want our country to be a success.
“They don’t want our country to exist.”