The Prime Minister will be absent when Ed Miliband and four other party leaders take part in a televised showdown - after agreeing to just one direct confrontation with his competitor for No10.
As Nicola Sturgeon heads to London for the broadcast, he will travel north of the border with a warning the SNP and Labour could join forces to pose “a clear threat” to the future of the United Kingdom.
The Scottish nationalist leader insisted her party - which opinion polls have on course to snatch dozens of seats from the Opposition - wanted a “progressive alliance” to bring an immediate end to austerity.
Mr Miliband will face similar demands from Plaid Cymru’s Leanne Wood and the Green Party’s Natalie Bennett and be forced to deal with attacks from the right by Ukip leader Nigel Farage.
Ms Wood said: “Surely Labour would rather accommodate such sensible plans than make this a decade of damaging Tory rule?”
But the Opposition leader, who has ruled out a formal coalition with the SNP but could rely on its support to prop up a minority government, will warn votes for rivals would result in a Tory victory on May 7.
Liberal Democrat leader Nick Clegg, also out of the BBC “challengers” debate as Deputy Prime Minister in the coalition Government, will be campaigning in north west England.
Launching the Conservatives’ Scottish manifesto, Mr Cameron will argue that the nationalists are “really on the same side” as Ed Miliband’s party when it comes to increasing spending and 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.”
Mr Balls will use a speech in the West MIdlands to accuse the Tories of “taking the British people for fools” with a string of “panicky promises” they could not afford without deeper cuts.
He will say: “It’s time for the Tories to come clean and explain: Where is the money coming from? How will you pay for your panicky promises? Who will pay the price?
“The Tories think they can get away with ducking these questions. It’s no wonder David Cameron isn’t turning up the debate tonight and has done everything he can to avoid a head-to-head debate.”
Despite Mr Cameron’s attempt to appeal to ordinary voters “it will be working families who end up paying the price again if the Tories win the election”, he will say.
“The Tories are taking the British people for fools and they shouldn’t be allowed to get away with it.”
Ms Sturgeon said the debate was “a fantastic moment of opportunity to deliver the positive change that people are looking for”.
“The SNP will be a positive and constructive voice at Westminster, ready to join others in a progressive alliance to end austerity and protect vital public services like the NHS.”
Calling on Mr Miliband to “change direction”, Ms Wood said: “We’re calling for an end to austerity and instead balancing the books through job creation and infrastructure investment.
“It’s time weapons of mass destruction were confined to history and we divert funds to improve standards of living and prospects for people.”
In the debate, each of the five leaders will be allowed a short opening statement before David Dimbleby invites questions from 200 voters.
Mr Miliband will be positioned to the far left next to Ms Wood and Mr Farage to the far right next to Ms Sturgeon with Ms Bennett in the middle of the group.
The 90-minute programme will cover five subjects. Each question will see one-minute opening statements followed by 10 minutes of free debate.
Tory deputy chief whip Greg Hands said: “Will we see the alternative to David Cameron in the Challengers Debate? A Frankenstein coalition to bankrupt and break up Britain.”
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