Nick Clegg has claimed that voters could be forced back to polling booths before Christmas - if other parties lock the Lib Dems out of government.
The Deputy Prime Minister raised the spectre of a second general election as he hit out at other smaller parties, including the SNP.
Despite predictions the Lib Dems could lose half their seats on Thursday night, Mr Clegg suggested that if they were not part of the next administration the resulting chaos would almost certainly lead to another election before the end of the year.
Opposition parties described the claims as ludicrous and said they revealed a panic at the heart of the Lib Dem campaign.
The SNP described the comments as "foolish".
Mr Clegg said that only a coalition involving his party could provide "stable" government.
Any attempt by the Tories or Labour to run a minority administration with smaller parties such as the SNP or Ukip would be "a shambles", he predicted, and force voters back to the polls.
Any move towards a second election, however, could face a potential stumbling in the Fixed Term Parliaments Act, brought in by the Lib Dems after the 2010 election.
Mr Clegg also confirmed that a referendum on the UK's membership of the European Union would not be a deal-breaker for his party
In recent days the Lib Dems have appeared to soften their stance on the issue, a move that could make it easier to do a deal with the Tories.
Mr Clegg said: "Everybody knows that no one will win this election - even if David Cameron and Ed Miliband won't admit it publicly
"The Liberal Democrats have shown that coalitions can be strong and stable. But instead of creating stability, Labour and the Conservatives will create a shambles.
"The last thing Britain needs is a second election before Christmas. But that is exactly what will happen if Ed Miliband and David Cameron put their own political interest ahead of the national interest."
In response Mr Cameron rejected his deputy's claim that the Tories could not win a majority at Westminster.
He said: "I have a very clear message for anyone in a Liberal Democrat-held seat ... and that is vote Conservative."
He also warned those who want a Tory-led government:"Nick Clegg was very clear at the weekend he is just as likely to support an Ed Miliband-SNP government ... as he is to support the Conservatives."
Angus Robertson, the SNP's election campaign director, said: "It is very foolish for the Liberal Democrats to be talking about another general election before people have had the opportunity to vote in this one.
"We are entirely focused on Thursday's election."
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