He argued that only another "stable and strong" coalition involving his party could save the country from a re-run.
He said "everybody knows that no-one will win this election", even if the two major parties fail to admit it.
"Politicians will have to work together to put the country first," he said.
Mr Clegg spoke out during a "two-day dash" from Land's End to John O'Groats in the final hours of the election campaign.
He said the hardening position of Labour and the Conservatives on coalition deals meant a minority government after 7 May would be more unstable - and "controversial" policies would cause it to fail.
"Everybody knows that no-one will win this election - even if David Cameron and Ed Miliband won't admit it publicly," Mr Clegg said during a campaign visit to Cardiff Central where the Lib Dems are fighting off a Labour challenge for the seat.
"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.
"If they try to stagger through with a messy and unstable minority government instead of putting the country first, then they will risk all the hard work and sacrifices people have made over the last five years.
"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."
Earlier Mr Clegg told BBC Radio 4's Today programme he was not in principle opposed to a referendum on EU membership - claiming other issues like the NHS and "balancing the books" were bigger priorities than the EU poll.
He also predicted his party would do "better than people think" on 7 May.
'I don't run a sect'
Insisting his MPs could be "the guarantors of stability at a time of great uncertainty", he insisted: "We'll do no deals, no pacts, no arrangements with either UKIP on the right or the SNP on the left.
"The great risk of not having the Liberal Democrats in Parliament, in government, is we lurch off into this blue-KIP alliance between David Cameron, UKIP, the DUP and the rest of them cutting, cutting, cutting way beyond what's necessary, or reckless borrowing as Ed Miliband and Alex Salmond are in some kind of deathly embrace on the left."
Main pledges
But Mr Clegg refused to be drawn on claims by Lib Dem peer Lord Scriven that David Cameron had privately told him before the election campaign that he did not believe the Conservatives would win a majority.
"In the same way I don't control the tweets of Liberal Democrats - I run a political party, I don't run a sect - I'm not going to provide a running commentary on private conversations," he said.
Describing the Conservatives as "panicky" about the outcome of the election, he added: "The claim they can win a majority by 23 seats is false - it's wrong - it's a big fat fib. They need to win 323 seats and I haven't met a senior Conservative who privately has ever admitted to me that they can do that."
The best of BBC News' Election 2015 specials
The UK could be poised for a second general election by Christmas if either Labour or the Conservatives try to form a minority government after 7 May, Lib Dem leader Nick Clegg has warned.