Nick Clegg will today accuse his cabinet colleague David Cameron of being "absent, hiding and missing in action" over Europe.
After two weeks of drubbings in high-profile televised debates with the Eurosceptic Ukip leader Nigel Farage, the Deputy Prime Minister will attempt to move the focus onto Labour and the Conservatives.
He will call on supporters of both parties who are keen to stay in the European Union to back the Liberal Democrats in this year's European Parliament elections. Yesterday Mr Farage attempted to play down the threat his party posed to the Conservatives as he took part in a live phone-in.
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The MEP's party has been accused of eating into the Tory heartland and potentially handing a general election victory to Labour in 2015.
But Mr Farage insisted only a small number of his supporters would consider voting Tory and said one in five had never voted before. He also warned the war on drugs was not working, said the UK sent too many people to university and defended his attendance record at the European Parliament.
In a speech to the Welsh Liberal Democrat conference Mr Clegg will say "standing up for Wales also means defending Britain's place in Europe".
"I challenged Nigel Farage to debate Britain's membership of the EU so that, for once, people would hear both sides of the argument: in versus out."
But he will also admit he lost those debates.
"Are we going to win that argument in 120 minutes? No, sadly not.".
In a full frontal attack on the other two main Westminster parties he will add: "Where are the supposedly pro-European Labour party? Or the supposedly moderate Conservative leadership?
"The country is finally having it out about the single biggest issue facing our future - and where are they?"
"Absent. Hiding. Missing in action."
Mr Clegg will add: "Paralysed by their own internal divisions. Too scared to speak up for the national interest because it'll cause them too great a political headache."