The prime minister says voters could end up with a Labour government propped up by a nationalist party who want to “break up Britain”.
At a campaign event to mark two months until the election, Mr Cameron said: “If you thought the worst outcome in this election is a Labour government led by Ed Miliband, think again.
“You could end up with a Labour government led by Ed Miliband, propped up by Alex Salmond and the Scottish National Party. You could end up with an alliance between the people who want to bankrupt Britain and the people who want to break up Britain.
“Even today, Ed Miliband will not rule out a deal or backing from the SNP.
“If he cares about this country, he should do so. You cannot let the people who want to break up our country into the government of our country.”
Mr Cameron added: “This election is a choice. And yes, there will be a lot of names on that ballot paper – you can vote Green, you can vote Ukip, you can vote Plaid Cymru, you can vote SNP, you can vote Lib Dem.
“But there are only two choices – there’s the Conservatives or Labour. There’s only two people who can end up as prime minister – me or Ed Miliband.
“We have got to be very clear about this choice.”
Scottish Labour leader Jim Murphy told a one-day party conference in Edinburgh that a vote for the SNP could help save David Cameron and lead to more “Tory austerity”.
He told activists: “In this forthcoming election there’s only one organisation that can save David Cameron in Scotland. That’s the SNP.
“The fact is that any seat that the SNP take from the Labour Party is an enormous step towards David Cameron clinging on to power in Downing Street in May.”
He added: “This is the closest election in my lifetime, the votes of Scotland will matter in a remarkable way in this election. All the recent polls shows it is all within the margin of error. So let’s make sure that Scotland never becomes the error in David Cameron’s margin.
“Let’s come together with a sense of pride and determination and let’s not inflict another Tory government and austerity on ourselves or the whole of the rest of the United Kingdom.
“If we don’t stand up to Tory austerity no-one else will. If Scotland doesn’t help prevent Cameron from getting into Downing Street, think of what it means for those families who are struggling to get by on foodbanks.”
Former Conservative chairman Lord Baker of Dorking said a Tory-Labour coalition could be needed to prevent the SNP from holding the balance of power at Westminster and ensure the “continuing unity” of the UK.
The former education secretary said such an agreement could avoid the “nightmare” of a minority Labour administration depending on SNP support to govern.
Scottish Labour leader Jim Murphy said: “I don’t need lessons from Tory dinosaurs about how to run Scotland. What a ludicrous, ludicrous idea. That’s never going to happen.”