Liberal Democrat leader Nick Clegg said his party would not back a Conservative government which attempted to impose swingeing spending cuts in an emergency budget.
Tory leader David Cameron has said his party would hold an emergency budget within 50 days as part of a programme to get the £167 billion deficit under control.
With the possibility of a hung parliament after the May 6 poll, Mr Clegg said his party would not lend its support to big cuts "at a time when the economy is struggling to get to its feet".
In an interview with Jeremy Paxman on BBC1, Mr Clegg said he accepted the need for some "big, long-term" decisions on public spending.
But, he said: "Do I think that these big, big cuts are merited or justified at a time when the economy is struggling to get to its feet? Clearly not."
He added: "Of course I would vote against cuts which would destroy any chance we would have of having a sustainable recovery."
But Mr Clegg - who would play a pivotal role in deciding which party forms the next government if one party does not achieve a majority in the General Election - did not rule out serving with either Mr Cameron or Gordon Brown.
He said: "I'm totally confident of the fact that if the British people say 'none of you guys deserve an outright majority' then the Liberal Democrats would play a responsible role in making sure that people get the kind of government that this country deserves."