The Liberal Democrat leader said a Labour or Conservative minority administration would only be able to pass important legislation with the support of Ukip, the SNP or the DUP and major “sweeteners” would be needed to maintain their backing.
Mr Clegg said: “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.”
The Lib Dems suggested Labour would have to concede full autonomy over tax and spending to Scotland and water down its commitment to Trident for SNP support to pass a Queen’s Speech and key budget votes.
The party also claimed the Conservatives would need to commit to an EU referendum this week and find £1bn for Northern Ireland to keep their support.
Mr Clegg is in the middle of a Land’s End to John O’Groats tour as part of a last effort to swing votes to the Lib Dems in key seats across the country.
He was buoyed personally by a new poll on Monday night which suggested tactical voting by Conservative supporters in his Sheffield Hallam constituency should save his seat.