"We have to act together if we want to make Britain safer, if we want to safeguard jobs and protect the environment," Mr Clegg - who was an MEP for five years before being elected to the House of Commons in 2005 - said.
"We are stronger together. We are poorer apart. It is as simple as that."
The Lib Dems were the only party which could get out of Europe "what British families need", he stressed.
While the EU was "far from perfect", Mr Clegg added, it was an organisation with enormous economic power which could offer the UK support and protection through the recession.
He said Labour was deflated and disunited while a vote for the Conservatives next month would be a "wasted" one as they were on the sidelines of the European debate.
"The Tories want to pull down the drawbridge, turn the clock back and retreat to the fanatical fringe of European politics."
David Cameron is pulling the Conservatives out of the European People's Party grouping of centre-right parties in the European Parliament, saying it is a federalist body.
The Tories intend to form a new grouping in the European Parliament after the election but the move has attracted criticism from some European leaders and Conservative MPs.
The Lib Dems' centralised, undemocratic vision of Europe belongs to the past
William Hague, shadow foreign secretary
The Conservatives said it was the Lib Dems who were isolated on Europe in their opposition to a referendum on the Lisbon Treaty and their support for eventual adoption of the euro.
"The Lib Dems' centralised, undemocratic vision of Europe belongs to the past," said shadow foreign secretary William Hague.
"We need a modern, flexible Europe that lets European countries deliver real benefits by working together and listens to people's views."
The Lib Dems trailed in behind the UK Independence Party in 2004, with 14.9% of the popular vote.
Politicians from the three major UK parties have expressed concerns that voters could use elections for the European Parliament to punish them for the expenses row bedevilling the House of Commons.
But a Populus opinion poll in Tuesday's Times, conducted by phone after the expenses details were first published, suggests the Lib Dems are now on 22% (up 4% on the month), with Labour on 26% (down 4%) and the Conservatives on 39% (down 4%).
Next month voters from across Europe will elect 736 MEPs, made up of representatives from all 27 EU countries. The UK is allocated 72 of these seats.
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