Two weeks ago he said that the chances of a second election were "remote" and "very unlikely".
He said that the new rules on fixed-term Parliaments made unseating a government harder, and that the public did not want an "endless cycle" of elections.
Mr Clegg appears now to have changed his mind.
Why? Well, he says that the hardening position of Labour and the Conservatives on coalition deals means that a minority government after the next election would now be more unstable.
Senior Liberal Democrat sources pointed out that Labour leader Ed Miliband had said on the BBC's Question Time last week that he wanted to push through "100%" of his manifesto.
That would make coalition difficult, the Lib Dems say.
One Lib Dem figure also argued that the Conservatives' claim that they were still pushing for a majority of MPs on 7 May was a "laughable denial of reality".
The Lib Dems claim that the Scottish Nationalists would demand an end to austerity and would want full fiscal autonomy for Scotland in return for their support for a minority Labour government.
Mr Clegg said that UKIP would want to cut overseas aid and would want a referendum on Britain leaving the European Union this year as the price for supporting a Conservative government.
He claimed that such "controversial" policies would cause a minority government to fall.
"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.
"That means that politicians will have to work together to put the country first.
"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 best of BBC News' Election 2015 specials
Liberal Democrat leader Nick Clegg has significantly changed his position on the risk of a second election this year.