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The Liberal Democrats will not enter into another coalition government unless an emergency "stability budget" is held within 50 days of the 7 May election, Nick Clegg has insisted.
His demand would effectively veto the Conservative's plan to cut £12bn from the welfare budget to balance the books if they went into coalition again.
He said the economic recovery would not have happened without the Lib Dems.
Labour wants to see the national debt falling in the next Parliament.
Mr Clegg told a morning press conference that the economy was the Lib Dems' second "red line" issue in any post-election deal - the first being a pledge to boost education funding in England to £55.3bn.
"I choose my words very carefully," he said. "If we don't have, in any coalition agreement which we enter into, properly funded nursery schools and colleges... and a stability budget within 50 days to keep the economic recovery and economy on track, Liberal Democrats will not enter government."
In a direct address to the leaders of the two major parties, Mr Clegg said: "David Cameron, Ed Miliband - the Liberal Democrats won't let you bluff your way through. We won't let you risk our economic recovery.
"We will have a stability budget, to take place within 50 days of election day, a pre-condition of any coalition arrangement.
"There will be no deal if there is no stability. No coalition without coming clean to the British people. This too is a red line."
He told reporters this was also a guarantee to the British public that the government would balance the books "in a timely fashion and in a fair way, and a clear commitment to our world class services".
Mr Clegg has previously condemned Conservative plans to balance the budget by cutting £12bn from welfare without increasing taxes on the wealthy.
He has also been critical of Labour for failing to set out a clear timetable for eliminating the deficit.
Instead, the Lib Dem leader said his plan would "calm jittery markets, keep interest rates low, keep Britain on track and show the British people how we will finish the job fairly and continue to support our public services".
Arguing that the Lib Dems would keep both major parties "on the straight and narrow", he added: "We won't let Labour risk your job or your economy with reckless borrowing. And we won't let the Conservatives risk our schools, hospitals and public services with reckless cuts."
Mr Clegg was also asked how his party would deal with the Conservatives' position on Europe, should the Liberal Democrats resume their role as coalition partners.
"My experience of being in government with them over the last five years is they change their position on Europe with dizzying regularity," he said.
"I have no idea what the Conservative Party is going to think about that next Tuesday, let alone on May 8."
On Monday, Mr Clegg claimed raising education funding in England from £49bn to £55.3bn over the next Parliament was the price of the Lib Dems entering coalition after the election.
He said he would "not accept under any circumstances the cuts to nurseries, to schools and to colleges that both Conservatives and now Labour have announced".
"And if we don't get that we wouldn't enter into a coalition in the first place," he continued.
"We are the only party to protect from cradle to college, from nursery to 19-year-olds.
"In pounds and pence - per year - we will be spending £2.5bn more than Labour, £5bn more than the Tories. That is a significant difference."