The Tory leader made a direct appeal for Ukip and Liberal Democrat supporters to cast their ballots tactically and keep him in Downing Street.
Mr Cameron insisted he is the “only option” for avoiding a “calamitous” and “chilling” tie-up between Ed Miliband and the SNP.
The plea, in a speech to activists at Ambleside Sports Club in Nuneaton, comes with the polls still showing the Conservatives and Labour effectively deadlocked.
Mr Cameron pleaded for voters to use the long Bank Holiday weekend to “stop and think” about the danger they face.
“It’s no exaggeration to say Britain will be making an historic choice,” he said. “Ed Miliband wants us to take a sharp turn to the left. He sincerely believes more borrowing and more taxes will help you and your family. He is sincerely wrong.
“Worse - because he will lose so many seats to the SNP in Scotland - he cannot form a majority Government on his own.
“He keeps trying to deny it - but it is a fact that he can only become prime minister with SNP support.
“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, for you and your family.”
Hoping to capitalise on his relatively strong personal ratings, Mr Cameron asked those thinking of voting Ukip or Lib Dem to switch to the Tories in order to keep him in Number 10.
“If you have got a view on who you would prefer as your Prime Minister express it at the ballot box,” he said.
“The outcome will not be decided any other way.
“Do not risk voting for another party and hoping that is the outcome. If you want your preferred Prime Minister get out there and vote for it.”
He warned that Ukip was a “back door to a Labour Government”, and suggested Nick Clegg was ready to be part of a “minority Labour Government, propped up by the SNP”.
The Conservatives have used their latest election broadcast to press home apocalyptic warnings about the threat of SNP influence.
In the film, the economy, represented by a clock, is smashed by a sledgehammer that represents the nationalists.
Mr Cameron then appears and again brandishes the note Liam Byrne left on his Treasury desk after Labour’s 2010 election defeat, which read: “Dear Chief Secretary, I’m afraid there is no money. Kind regards - and good luck! Liam.”
The PM adds: “Vote for the Conservatives on Thursday and you will have the security of a plan that is working. You’ll have stable government and the strong economy on which our schools, our NHS and your family’s future depend.
“But vote for any other party and Britain’s recovery could be stopped dead in its tracks.”
Leader of the Commons William Hague has compared the position of the SNP with that of Irish nationalist parties in the early 20th century. They won increasing numbers of Westminster seats in the years before Ireland won its independence in 1921 and held the balance of power after the 1910 general election, securing a home rule bill as a result.
“Maybe there are parallels,” the former Tory leader and history writer told Sky News’ Murnaghan.
“There were a huge number of Irish nationalists elected to Parliament in the 19th century and the early 20th century and that did distort British politics for a long time. It’s certainly not good for the UK as a whole.
“What would happen in this situation, if there was a weak Labour government held to ransom by Scottish nationalists is that those nationalists would try to divide Scotland against England with everything they did every day.”
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