The Tories have nothing meaningful to say on Europe, Deputy Prime Minister Nick Clegg claimed as he sought to portray May's European elections as a straight choice between his Liberal Democrats and Ukip.
Mr Clegg has gambled that casting the Lib Dems as the "party of in" will appeal to voters concerned about severing ties with the European Union.
The Lib Dem leader's intervention came as Business Secretary Vince Cable claimed the Tory policy on Europe, which involves an in/out referendum by the end of 2017, was "very irresponsible" and posed a threat to the economy.
In an interview with The Guardian, Mr Clegg said: " The two main parties in this country no longer have anything meaningful to say about Europe.
"Labour seems to be enjoying sitting on its hands and in a rather cowardly manner failing to make the internationalist case at all.
"The Conservatives are deeply divided between a significant part that agrees with Ukip, and another part casting around for ideas to keep them in the EU, so Labour and the Conservatives are split or confused, or completely lost the courage of their convictions.
"When big parties create vacuums like that they get filled and they are being filled by the two other parties.
"Whether you agree with us or not, at least we have the merit of having clear and consistent views on this."
Mr Clegg will take on Ukip leader Nigel Farage in two head-to-head debates ahead of the European elections as part of the Lib Dem effort to set them up as the natural opponents to the party which has pushed them into fourth place in a series of opinion polls.
He said: " The irony is that you have got the two smaller parties telling the country what the choice is, and the choice increasingly is whether we remain in or not.
"It is the same choice in Dutch, French and German politics - do you respond to globalisation by being open or closed?"
Mr Clegg said he understood the appeal of Ukip to voters: " At a time of economic insecurity, in the wake of this massive cardiac arrest we suffered back in 2008, because of all of the anxieties associated with globalisation where people feel their lives are being affected by forces which they can't control, I think it is totally logical actually that some people will find it quite seductive when a politician says, 'it's someone else's fault and all your anxieties can be lifted from your shoulders if only we just shut the door'."
In contrast to David Cameron, he added: "I've never dismissed Ukip as nutters and fruitcakes.
"I take what they represent deadly seriously and, by the way, I wouldn't be having a debate with Nigel Farage unless I took what he says very, very seriously."
But he said Mr Farage "wants to turn the clock back, he doesn't like modern Britain, he wants to dismiss so many of the big modern features of modern life - I want to embrace them".
Both Ukip and the Eurosceptic wing of the Tory party came under fire as the Lib Dems' spring conference got under way in York.
Mr Cable w arned that Eurosceptics were putting the union with Scotland at risk as well as potentially damaging business.
He said: "Nick has been absolutely right to take on Ukip and the Eurosceptic wing of the Tory Party who are potentially doing so much damage by creating uncertainty, blight over future investment and potentially stopping the recovery which this country very badly needs ... There are about 3.5 million people in this country whose jobs depend on that market and for the Tories now to be raising doubts about it is very, very irresponsible."
Turning to the impact on the Scottish debate, ahead of September's independence referendum, he added: "How on Earth do you expect to persuade the Scots to ignore the siren voices of nationalism and separatism when you indulge in British nationalism and Euro-separatism?"
The conference continues tomorrow with activists set to consider a new set of immigration policies - replacing the Lib Dems former commitment to an amnesty and the potential for regional quotas.
Instead activists will be asked to back a push for European migrants coming to the UK being forced to wait six months before gaining access to the benefits system as part of a Europe-wide reform.
But the changes proposed by the Lib Dems would also make it easier for immigrants to bring members of their family to join them in the UK.
Mr Cable, who has repeatedly clashed with coalition colleagues over immigration, is expected to request to take part in the debate along with rank and file party members.
Coalition welfare policies will also come under scrutiny as the Lib Dems debate a motion calling for an urgent independent review into the relationship between benefit mistakes or penalties and the growth of food poverty.
The scale of the electoral challenge facing Mr Clegg's party was underlined after the Lib Dem standing in a city council by-election in the Clifton area of Nottingham failed to secure as many votes as a candidate dressed as Elvis Presley.