European elections: Clegg urges Lib Dems to combat 'EU myth-makers'
Nick Clegg has pitted his party's "optimism and openness" against the "fears and falsehoods" of isolationists at the launch of the Liberal Democrat campaign for the European elections.
His said the UK Independence Party's plan to leave the EU was a "dangerous fantasy" which would cost British jobs.
But he also criticised Labour and the Conservatives for going "missing in action" instead of challenging UKIP.
Only Lib Dems would fight for the "best of British values", the deputy PM said.
The European elections take place on 22 May, with the Lib Dems widely predicted to give a worse showing at the ballot box than in 2009.
Mr Clegg, whose party is recognised as the most pro-European of the largest three at Westminster, recently took part in two televised debates on the issue with UKIP leader Nigel Farage.
Mr Clegg, who served as a member of the European Parliament from 1999 to 2004, launched his party's campaign in Colchester, Essex, where he was introduced by the local Lib Dem MP Sir Bob Russell.
The Lib Dem leader said: "For far, far too long, the isolationists have got away with peddling their myths, their fears, their falsehoods, without any challenge whatsoever, pretending that every problem in the world would somehow disappear, like the morning mist, if only we were to pull ourselves out of the EU.
"It is a dangerous fantasy, because as night follows day, I can tell you without any fear of contradiction that if we were to pull ourselves out of the EU, there would be more people out of work."
He compared leaving the EU to an "act of monumental economic vandalism".
"That is why we should now draw a line in the sand as the leading party of in in British politics, and say to the isolationists, to the myth-makers, to the scaremongers: 'Enough!'
"David Cameron and Ed Miliband are missing in action, sitting on their hands, scared of their shadows, frightened of their own internal party divisions, unwilling to make the case for an open, outward-facing, engaged Britain in the modern world.
"So we have to do it, and we will do it, as the leading 'party of in' in these European elections."
The EU was not "perfect", he continued, and needed reform, just as Westminster and Whitehall need reform.
But Mr Clegg argued that remaining in the EU would safeguard millions of British jobs, help the police fight cross-border crime, and make it easier to "confront the biggest challenge of our era: that of global climate change".
"Being in means acting in accordance with the best of British values: open-hearted, generous, engaged," he said.
Mr Clegg concluded by telling party activists: "Battle has been joined. There is a choice to make. You have to pick sides.
"So I say to you: go out and campaign with all the gusto and determination that you can muster, go out to campaign against the politics of fear, falsehoods and pessimism of the other side, and instead go out and campaign for the politics we believe in, of optimism, of openness, and of jobs."
UKIP says leaving the EU will improve prosperity by cutting bureaucracy and that it will allow the UK to regain control of immigration.
The Conservatives are promising an "in-out" referendum on EU membership by the end of 2017 if they win the 2015 general election. But Labour has said such an event is "unlikely" if it is in government.